Flame Radicals Research Articles

Development of the detailed mechanism of pyrolysis and combustion of triphenyl phosphate: New quantum chemistry calculations and experimental data on structure of the H2/O2/Ar flame doped with TPP

A possible method for a flammability reduction of the polymeric materials is the introduction of flame retardants into their composition. An effective search for optimal flame retardants and an understanding of their inhibition mechanisms requires the extensive information on the chemistry of their transformation in flames. A new data on chemical flame structure of a premixed H2/O2/Ar flame doped with 1000 ppm of triphenyl phosphate (TPP) at a pressure of 1 atm was obtained with a molecular-beam mass-spectrometry technique. Among intermediate products of the TPP decomposition were identified: several large phosphorus containing compounds, small phosphorus containing species (PO, PO2, HOPO, HOPO2), cyclic hydrocarbons (benzene, toluene, phenol), phenyl and phenoxy radicals. The detailed kinetic mechanism proposed earlier for the thermal degradation of TPP was updated with several new reactions including reactions with common flame radicals H/OH/CH3. Reaction rate constants were calculated using the Rice-Ramsberger-Kassel-Marcus theory and potential energy surfaces obtained by the DLPNO-CCSD(T)/cc-pVQZ//ωb97xd/6-31G(d) method. A comparison between experimental data and simulation results with the new model has shown a satisfactory qualitative and quantitative agreement, which confirmed that, the TPP decomposition occurs in flame according to the proposed scheme.

Toward better halon substitutes: Effects of carbon chain length on pyrolytic and fire-suppressing mechanisms of perfluoroalkanes

The use of halon fire extinguishants has led to the depletion of the ozone layer, prompting numerous international organizations to invest significant efforts in finding suitable halon alternatives. Among the counterparts, perfluorinated compounds stand out due to their environmental friendliness, specifically, CF3CF3, CF3CF2CF3 and CF3CF2CF2CF3 have been identified by NASA as the most promising candidates within this chemical family. This study digs into the impact of carbon chain length on thermal decomposition and fire suppression mechanisms of perfluoroalkanes with the three agents as model substances, employing experimental and theoretical analyses. Thermal decomposition products within the temperature range of 600 °C - 900 °C and fire suppression performances, including fire-extinguishing concentrations (9.38 vol.%, 7.81 vol.%, and 6.87 vol.%, respectively), variation of the flame morphology and temperature, are intensively analyzed and compared for the three perfluoroalkanes. Theoretical calculations indicate a decrease in carbon chain stability with the increasing chain length, and CF3CF2CF2CF3 manifests the most diversified self-decomposition paths at low energy barriers. Moreover, a wider spectrum of free radicals can be generated from the longer carbon chain via the CC cleavage, which can efficiently react with and continuously consume flame radicals (H• and OH•) to interrupt the combustion chain reaction. These findings enrich the structure-performance relationship of perfluoroalkanes, highlight the potential of specific perfluorinated compounds as viable halon alternatives and offer theoretical guidance for the selection and design of environmentally benign and efficient halon substitutes.

Identifying the regularities of n-heptane flame inhibition by inorganic compounds

The task related to using inorganic compounds for extinguishing flames is to enable their inhibitory capacity during operation within wide limits. Therefore, the object of the current research was aqueous solutions of inorganic salts, on which the effectiveness of inhibitory properties during interaction with n-heptane flame was established. It has been proven that increasing the mass flow rate of water by 1.5 mg/s reduces the intensity of OH-radicals radiation from 70 % to 30 % and lowers the flame temperature by 90 °C. However, it was found that when potassium salts are given, the intensity of OH-radicals radiation decreases by more than 6 times, potassium chloride and sulfate reduce the intensity of OH-radicals radiation by more than 2.8 times. Among ammonium salts, salts of dihydrogen phosphate and ammonium hydrogen phosphate reduce the relative intensity of radiation of OH radicals by more than 1.3 times. Sodium salts include nitrates and sodium chloride, which reduce the relative radiation intensity of OH radicals by more than 1.6 times. This is manifested, first of all, in the enrichment of the combustible environment with fuel. When determining the flame temperature of flammable liquids, it was established that n-heptane has the most stable and highest flame temperature, which is 1768 °C. When adding inorganic compounds to the flame of n-heptane, nitrate salt, and potassium chloride, the flame temperature increased by less than 20 °C. However, ammonium salts increased the flame temperature to over 140 °C, despite the presence of water. The practical significance is that the results were taken into account during the design and development of extinguishing agents for extinguishing fires. Therefore, there are reasons to assert the possibility of regulating flame extinguishing processes by using inorganic compounds capable of inhibiting active flame radicals

Quantitative 2D reconstruction of temperature and OH concentration in axisymmetric flames based on ultraviolet broadband absorption tomography

Absorption tomography plays a crucial role in quantitatively imaging temperature and absolute concentration in combustion. However, there is scarce research on the tomography of OH radicals, which are significant for combustion diagnostics. This study presented a new tomography method to image OH radicals, termed ultraviolet broadband absorption tomography (UV-BAT). Three strategies were proposed to solve the nonlinear problem in UV-BAT tomography, namely all-spectra fitting, weak-absorption approximation, and multi-temperature approximation. The proposed UV-BAT method was applied for OH imaging in methane/air axisymmetric laminar Bunsen flames with different equivalence ratios (ϕ = 0.9, 1.0, and 1.1). The reconstructed OH distributions showed distinctive structures of Bunsen flames including premixed and diffusion flame areas. The accuracy of the UV-BAT method was validated by comparing tomography results with computational fluid dynamics (CFD) simulations using the USC-Mech II mechanism, demonstrating good agreement in two-dimensional (2D) temperature and OH concentration distributions. This novel method offers new opportunities for tomographic reconstruction of free radicals such as OH and temperature in small-scale flames.Novelty and significanceThis study demonstrated a novel tomography imaging method using ultraviolet broadband absorption spectroscopy to reconstruct temperature and radical concentration in combustion. This method possesses high sensitivity to free radicals and employs compact and inexpensive equipment, making it one of the most viable methods for realizing free radical absorption tomography. The 2D imaging of OH radicals using this method based on axisymmetric absorption tomography was accomplished for the first time. In conclusion, this study enriched the diagnostic methods of flame radicals based on absorption spectroscopy and provided new ideas for small-scale temperature tomography.

Ammonia blends for gas-turbines: Preliminary test and CFD-CRN modelling

Ammonia, with its high hydrogen density, cost effectiveness and ease of storage, is emerging as a potential fuel to meet decarbonisation targets in various sectors. However, the combustion of pure ammonia poses challenges due to its low calorific value and reactivity. Existing gas turbine combustion systems, refined over decades with conventional fuels, struggle to maintain comparable flue gas emission concentrations when burning ammonia.Overcoming this challenge requires a paradigm shift in gas turbine combustion, with implications for chemistry and combustor design methodology. Research into the chemical kinetic mechanism for ammonia combustion worldwide reveals differences in flame radical pools compared to methane combustion. The dominant NOx formation mechanism shifts to a "fuel-bound nitrogen" type, which differs from the "thermal" mechanism in hydrocarbon and hydrogen combustion.The integration of chemical reaction rates into Computational Fluid Dynamics (CFD) models with transport equations results in impractical computational times for industrial applications. Therefore, streamlined approaches such as chemical reaction reduction methods are being developed. This involves the construction of lumped parameter models (Chemical Reactor Network - CRN) to identify key parameters that affect species production. A reliable CRN implementation involves the identification of critical volumes using a preliminary CFD approach (CFD-CRN).This paper analyses preliminary experimental tests by Baker Hughes on an industrial burner using different ammonia, hydrogen, and methane fuel mixtures. The NOx results show a different behavior between methane and hydrogen. While the detailed mechanisms behind these results remain open questions, the CFD-CRN modelling approach is adopted. A proposed CRN is investigated for methane, hydrogen, and ammonia blends, providing insight into key parameters and a reasonable physical explanation for the different NOx results. A single calibration parameter (ϕpilot/ϕref) is used to predict accurately NOx emissions using experimental data available from experimental campaign.

Structure and nitric oxide formation in laminar diffusion flames of ammonia–hydrogen and air

In this work, we present an investigation of the structure and formation of nitrogen oxides (NOx) in an axisymmetric laminar diffusion flame in which an ammonia–hydrogen fuel stream is surrounded by co-flowing air. Green ammonia, produced using renewable energy, is a promising carbon-free fuel for replacing hydrocarbon fuels over a diverse range of combustion applications. A key challenge for ammonia combustion is to understand the kinetics of nitrogen oxide formation for designing low-NOx combustion systems. In this experimental study, ammonia fuel is blended with hydrogen to increase fuel reactivity, with a fuel composition range of 15%–100% hydrogen. NOx emissions and fuel slip are measured by downstream sampling of NO, NO2, N2O and NH3 with a Fourier Transform Infrared (FTIR) gas analyzer. Flame structure is visualized by planar laser-induced fluorescence (LIF) for NO, NH, and OH radicals, as well as OH* by filtered chemiluminescence. Species concentration profiles are compared to predictions from 2-dimensional axisymmetric numerical models of the laminar flame using recent kinetic mechanisms developed for ammonia combustion. No measurable ammonia slip was recorded for any fuel composition, in agreement with model predictions. Good agreement between predictions and measurements was obtained for exhaust nitrogen oxide concentrations and for in-flame radical profiles, although larger variation and deviations are observed for predictions of NO2 and N2O than for NO. Numerical predictions for spatial distribution of flame radicals and flame liftoff height also showed good agreement with LIF and chemiluminescence measurements. For high fuel hydrogen content, the measured NH and NO profiles are shifted inward toward the fuel outlet, away from the peak temperature contour. Reaction path analysis is used to illustrate the important contributions to NO creation and destruction, including the differing role of key reaction pathways with varying fuel hydrogen content.

Impact of pilot flame and hydrogen enrichment on turbulent methane/hydrogen/air swirling premixed flames in a model gas turbine combustor

This work investigates the impact of pilot flame and fuel composition on the structures and stabilization of swirling turbulent premixed methane/hydrogen/air flames in a lab-scale gas turbine model combustor. Simultaneous measurements of the velocity field and OH radicals distribution in the combustor were conducted using particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) methods, respectively. Flames under stable and close to lean blow-off (LBO) conditions were studied for two fuel mixtures, with a hydrogen mole ratio of 0 and 50 % in the hydrogen/methane mixture, respectively. The studied flames were at a constant Reynolds number of 20,000 with different equivalence ratios. Two pilot-to-global fuel ratios were investigated (2 % and 6 %) while keeping the pilot-to-global air ratio constant at 2 %. Data for non-piloted flames were also acquired for comparison. The pilot flames were shown to extend the operability range. The LBO equivalence ratio of the main flame decreased with increasing fuel mass flow rate in the pilot flames due to the increased amount of hot gases with high concentrations of OH radicals in the outer recirculation zone (ORZ), which significantly enhanced the stabilization of the main flame. The stable flame reaction zone was in the high-speed shear layer between the ORZ and the inner recirculation zone (IRZ). When approaching LBO, the reaction zone was pushed downstream to the IRZ and subsequently decreased the size of IRZ, indicating a strong flow/flame interaction. Hydrogen enrichment was shown to reduce the LBO equivalence ratio. When close to LBO, the OH radicals in the hydrogen-enriched flames were observed in isolated pockets due to differential diffusion, which enhanced resilience to LBO. The flame front curvature, mean progress variable, and flame surface density were calculated from the acquired OH-PLIF data to quantify the impact of fuel composition and pilot flames on the flame structures.

Dry water consisting of nano SiO2 modified by fluorinated surfactant: Applied to AlH3 explosion suppression

AlH3 has a high explosion risk due to rapid hydrogen evolution, and explosion suppression is necessary to reduce explosion hazard. Herein, we apply a common fire extinguishing agent, fluorinated surfactant, to suppress hydrogen in AlH3. This stable composite suppressant has a structure resembling dry water, a dilute aqueous solution of FS-50 encapsulated by fumed phase SiO2, which not only improves suppression effect but also significantly reduces the damage of fluorine to the environment. Fluorinated intermediate radicals F, CF, CF3, CHF, CHF2, etc. produced by decomposition of FS-50 interrupt the chain reaction of flame radicals H, O, OH to produce (-CH2-CF2-)n and stable product HF. Additionally, the outflow of fluorinated compounds mixes with water and SiO2 to form a cladding layer, which prevents heat propagating from the reaction zone to preheating zone. Our work will pave the way for the widespread and safe application of light metal hydrogen storage materials.

Flame Structure at Elevated Pressure Values and Reduced Reaction Mechanisms for the Combustion of CH4/H2 Mixtures

Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions.

Effect of oxidation on the combustion flame characteristics of Jatropha biodiesel

This study investigated the effect of oxidation on the pyrolysis and combustion flame performance of Jatropha biodiesel. To analyze the escape characteristics of multi-component gas products during the pyrolysis and high-temperature oxidation processes of fresh and oxidized Jatropha biodiesel, the TG-FTIR-MS combined system was employed. In addition, various analytical equipment such as an OH-PLIF system, a flue gas analyzer, a spectral analyzer, and a transmission electron microscope were employed to examine the laminar premixed combustion flame and pollutant emissions of Jatropha biodiesel before and after oxidation. The results demonstrate that oxidation exhibits a nonlinear effect on the thermal decomposition of Jatropha biodiesel. Notably, the reaction activation energy exhibits an initial increase and followed by a decrease. Moreover, an increased oxidation degree led to a reduction in the activation energy of the high-temperature oxidation reaction of Jatropha biodiesel, without considerably affecting the total weight loss temperature. Furthermore, the peak temperatures of DTG and DSC exhibited a minor decrease. The production of C2, CH, and OH free radicals in the combustion flame of oxidized Jatropha biodiesel increased, along with an elevation in the spectral intensity of carbon smoke. With an increase in the degree of oxidation, the CO emission concentration from the combustion of Jatropha biodiesel decreased gradually from 75 mg/m3 to 36 mg/m3 after 6 h of oxidation. Additionally, NOx emissions gradually increased from 131 mg/m3 to 186 mg/m3 after 6 h of oxidation, and the particle size of carbon smoke exhibited a reduction of 28%.

A comparative on the thermal decomposition and fire‐extinguishing performance of C5F8 and C5F7Cl

AbstractCompared with fluorinated cycloalkanes and liner olefins, the degradation of chlorofluorocycloolefins is much easier with good environmental friendliness. However, the fire‐extinguishing performance of chlorofluorocycloolefins has been rarely explored. In this study, the fire‐extinguishing performance and working mechanism of octafluorocyclopentene (C5F8) and 1‐chloro‐2,3,3,4,4,5,5‐heptafluorocyclopentene (C5F7Cl) were studied by combined experimental and theoretical approach. It was found that, compared to C5F8, the asymmetry of C5F7Cl agent with the chlorine atom possesses a reduced energy barrier in pyrolysis process and its reaction with OH· radicals. Moreover, chlorine‐containing unsaturated active groups and CF2· radicals generated in C5F7Cl pyrolysis can effectively capture flame radicals. As a result, a relatively low minimum extinguishing concentrations (MECs) for suppressing methane‐air (5.37 vol%) and propane‐air flame (5.93 vol%) is achieved outperforms that of C5F8 (6.73 and 7.74 vol%), even better than reported fluorinated cycloalkanes and chlorofluoroolefins agents. This study shows that chlorofluorocycloolefins are promising Halon substitutes over perfluorocyclic olefins in fire‐extinguishing performance.

A novel dry water with perfluorohexanone for explosion suppression of AlH3

AlH3 is a promising candidate for hydrogen storage, but has high explosion risk. To prevent the accident of AlH3, the novel dry water (DW) was applied to the suppression of AlH3 explosion, and the addition of perfluorohexone improved the suppression effect significantly. When the mass fraction of 16.70wt% perfluorohexanone-modified dry water (PMDW) was added with 700 g/m3, the maximum explosion pressure decreased by 54.37 %, and the maximum explosion pressure rising rate dropped by 98.80 %. The 3-dimensional network structure of silica, adsorbing flame radicals and preventing heat transfer, was proved to play a pivotal role in the explosion suppression by HRTEM and NMR. The evaporation and decomposition of the internal solution would greatly improve the cooling effect. CF2 captured H, O, and OH radicals in the flame and interrupted the free radical chain reaction. Our work is a step towards the application of eco-friendly DW in the field of explosion suppression.

Propylene Oxide Addition Effect on the Chemical Speciation of a Fuel-Rich Premixed n-Heptane/Toluene Flame.

1,2-Propylene oxide (PO, C3H6O) is considered as a promising agent for improving fuel. In this work, the effect of PO additives on the species pool in a premixed burner-stabilized fuel-rich (ϕ = 1.6) flame fueled by n-heptane/toluene mixture (7/3 by volume of liquids) at atmospheric pressure is studied by the flame-sampling molecular beam mass spectrometry and numerical modeling in order to get insight into the chemical aspects of the influence of oxygenates with an epoxy group on the formation of abundant intermediates (including PAH precursors) during combustion of fossil fuels. The flames with various loadings of PO in the fuel blend (from 0 to 16.3% in mole basis) are examined, and detailed kinetic mechanisms available in the literature are validated against the measurements of mole fraction profiles of reactants, major products, and many intermediate species. A higher reactivity of the fresh mixture and a reduction in the peak mole fractions of intermediates playing an important role in PAH formation (benzene, styrene, ethylbenzene, phenol, acetylene, diacetylene, etc.) are observed when PO is added. This was found to be due to simultaneously two factors: the partial replacement of "sooting" fuel (toluene, which is the main precursor of these species) with oxygenated additive, and the changes in the flame radical pool caused by PO addition. Propylene oxide additive was found to change the ratio between H, OH, O, and CH3 toward an increase in the proportion of O and CH3. The detailed kinetic mechanisms considered in the work are found to overpredict the peak mole fraction of acetylene, a key species playing a crucial role in PAH growth. Its chemistry is revisited in order to provide a better prediction of C2H2 and, as a result, PAHs.

Study on cool flame radical index and oxygen concentration dependence of oxygenated fuels

The growing utilization of alternative fuels has sparked an interest in understanding the combustion characteristics of oxygenated fuels at low temperature. Cool flame extinction limits provide direct measures of the low-temperature reactivity of fuels and critical information for advanced low-temperature combustion engine design. This paper investigates the effects of fuel and oxygen concentrations on the extinction limits of diffusion cool flames for oxygenated fuels, including dimethyl ether, methyl decanoate, and 1-dodecanol in an atmospheric counterflow burner. The cool flame radical indexes of these oxygenated fuels are developed by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The results show that the ranking of low-temperature reactivities of long carbon chain oxygenated fuels, compared with n-alkane, is ether > n-alkane > alcohol > ester for a similar carbon number. Furthermore, due to the critical role of multiple oxygen addition reactions in the low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is also explored. The results show that the cool flame extinction limits of the oxygenated fuels are proportional to a nth power of the oxygen concentration, [O2]n, due to the combined effects of multiple oxygen addition reactions and the negative temperature coefficient in low-temperature chemistry. Additionally, the measured n number of 1-dodecanol is found to be larger than 2, which suggests the existence of the third oxygen addition reactions in low-temperature chemistry for large alcohols.

High-speed planar imaging of OH radicals in turbulent flames assisted by deep learning

High-speed planar imaging of OH radicals in turbulent flames assisted by deep learning

Experimental and kinetic modeling study of di-n-propyl ether and diisopropyl ether combustion: Pyrolysis and laminar flame propagation velocity

To explore the fuel isomeric effect on ether combustion characteristics, pyrolysis, and laminar flame propagation of di-n-propyl ether (DPE) and diisopropyl ether (DIPE) were investigated. A new kinetic model of DIPE was developed based on our recent DPE model [Fuel 298 (2021) 120797]. The pyrolysis experiments were carried out in two jet-stirred reactors at near-atmospheric pressure with good agreement on the measurements using synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography/mass spectrometry. The decomposition profile of DIPE showed a faster trend than that of DPE, which can be attributed to the faster alcohol elimination reaction of DIPE. The dominant roles of alcohol elimination reaction and H-abstraction reactions in fuel consumption explain the production of fuel-specific oxygenated species, i.e. n-propanol and propanal in DPE and i-propanol, acetaldehyde and acetone in DIPE. The laminar burning velocities of DPE and DIPE were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 373 K and pressures of 1–10 atm. It was found that the linear DPE propagated faster than DIPE with the branched structure under all investigated conditions. Rate of production analysis and sensitivity analysis were also performed to elucidate the key radicals and reactions responsible for the remarkable reactivity of isomeric fuels. Fuel structures have a great influence on the distribution of radical pools, resulting in the easy formation of active radicals in DPE flames such as vinyl and ethyl and stable radicals in DIPE flames like methyl and allyl. The successive decomposition reactions of these dominant radicals promote the DPE flame and inhibit the DIPE flame propagation respectively, which explains the higher laminar burning velocities and reactivity of DPE than that of DIPE. Furthermore, the present model was also examined against the literature data, including pyrolysis and oxidation in the jet-stirred reactor and flow reactor.

OH-Planar Laser-Induced Fluorescence Measurements in Laminar Diffusion Flames of n-Heptane with Coflow Air Diluted by N2 and CO2

The distribution of hydroxyl (OH) radicals in the laminar diffusion flame of n-heptane was studied by planar laser-induced fluorescence (PLIF). The influences of nitrogen (N2) and carbon dioxide (CO2) dilution on the formation and distribution of OH radicals were analyzed. The corresponding dilution ratios (volume fractions) of both N2 and CO2 vary from 0 to 5%. The results show that for the n-heptane flame, the OH radical is mainly concentrated in the two wings of the flame, presenting a radially approximate symmetrical distribution. Both N2 and CO2 dilutions decrease the intensity of the maximum OH radical fluorescence and the total OH radical fluorescence. Moreover, the flame temperature decreases more significantly with the CO2 dilution ratio due to the combination of the dilution effect and thermal effect.

Experimental and kinetic modeling study on NH3/syngas/air and NH3/bio-syngas/air premixed laminar flames at elevated temperature

Ammonia (NH3) can help achieve large-scale storage and long-distance transportation of renewable energy. The co-firing of NH3 with syngas and bio-syngas and increasing the initial temperature of reactants are economical and effective methods to enhance the reactivity of NH3 flames in boilers and gas turbines. An accurate kinetic model is essential for developing corresponding burners. The current kinetic models are not accurate enough in temperature dependence. Laminar burning velocities (SL) of NH3/syngas/air and NH3/bio-syngas/air mixtures were measured at various H2 contents in syngas and NH3 contents in the mixtures, equivalence ratios of 0.7~1.4, atmospheric pressure, and elevated temperature up to 423 K in a high-pressure constant-volume combustion vessel. A detailed kinetic model was developed for NH3/syngas combustion and validated based on the literature data. The experimental results show that slightly rich flame, high mole fractions of syngas/bio-syngas, and elevated initial temperature contribute to increasing the SL. Kinetic modeling analysis was performed for interpreting the effect of fuel composition, equivalence ratio and initial temperature on the laminar flame propagation characteristics. The elevated H2 content in syngas provides a stronger chemical effect and the enhanced laminar flame propagation of NH3/syngas/air and NH3/bio-syngas/air mixtures is dominated by chemical effect. Due to the presence of diluent (CO2 and N2), thermal effect has a negative influence on the enhancement of NH3/bio-syngas/air flame propagation. Although diluent hardly changes the NH3 consumption pathways, it will significantly reduce the NH3 consumption. Different NH3 consumption pathways are preferred by lean and rich flame. Sensitivity analysis shows that NH2 chemistry is favored by rich flames. Since NH2 + NH = N2H2 + H promotes the production of H radical in rich flames, it has the greatest influence on the SL of rich flames. H2 is more capable of improving the temperature dependence of NH3/syngas/air flame than CO. The increased importance of NH3 chemistry accounts for the strong temperature dependence of NH3/bio-syngas/air flame.

QUANTUM CHEMICAL SIMULATION OF THE INHIBITORY EFFECT OF AQUEOUS SOLUTIONS OF INORGANIC COPPER(II) SALTS ON THE COMBUSTION OF HYDROCARBONS

Introduction. The search for chemicals that would have an effective fire extinguishing effect and the development of new fire extinguishers based on them is an extremely important problem of fire safety. It is known from the literature that new aqueous fire extinguishing agents (AFEAs) based on dissolved inorganic salts of transition metals, in particular, copper(II) chloride salts, have a rather efficient inhibitory effect on the hydrocarbon flame. However, the mechanism of inhibition of hydrocarbon combustion by this class of substances is not completely ascertained. However, it is reliable information about the processes that take place in the flame after the bringing in there of the aerosol of the mentioned AFEA will allow a systematic search for more optimal chemical composition of dissolved inorganic salts of d-metals. Purpose. The purpose of the work is to reveal the peculiarities of the interaction of concentrated aqueous solutions of copper(II) chloride salts with chemically active flame particles.Methods. Quantum chemical calculations of the chemical activity of radicals that appear in the flame and the physicochemical processes that occur in the flame after the bringing on there of AFEA aerosol.Results. The mechanism of a fire-extinguishing effect of aqueous solutions of inorganic copper(II) salts on a hydrocarbon flame is investigated by a calculation method. The sequence of stages of chemical processes that occur in the flame during the inhibiting combustion of hydrocarbons by AFEAs—concentrated solutions of CuCl2 and K2[CuCl4]—and the thermal effects of all reactions that accompany each of these stepwise transformations were ascertained. The stages of the interaction of gaseous Cu2Cl4 molecules with ×OH and ×H radicals in flame with the formation of first a radical-molecular complex and then a molecular complex are decisive in the process of inhibition and display the processes of interruption of chain reactions, i.e. deactivation of radicals in a flame.Conclusion. Thus, using the method of quantum chemical calculations the mechanism of inhibition of hydrocarbon combustion by copper(II) salts was offered. The mechanism of this process is considered to be associative, the decisive elementary act of which is carried out according to the scheme of addition of active radicals of a flame (×OH particles) to gaseous molecules Cu2Cl4 with the formation of radical-molecular complex [{Cu(×OH)Cl2}2] and with its subsequent deactivation by ×H particles.

The radical index and the effect of oxygen concentration on non-premixed cool flame extinction of large n-alkanes

The effects of fuel and oxygen concentrations on the extinction limits of non-premixed cool flames for large n-alkanes, ranging from n-heptane to n-dodecane, are measured in an atmospheric counterflow burner. The measurements are used (a) to derive a scaling law between the cool flame reactivity and the oxygen concentration and (b) to develop a radical index to rank the cool flame reactivity of n-alkanes, with the assistance of kinetic modeling. First, due to the significant role of multiple oxygen addition reactions in the low-temperature branching sequence, the relationship between cool flame extinction (and hence the chemical time scale) and oxygen concentration are examined. The results suggest that all measured n-alkane cool flame extinction limits exhibit similar nonlinear dependence on oxygen concentration. The results further show that, due to the combined effects of multi-oxygen addition processes and the negative temperature coefficient effect, the cool flame low temperature reactivity of n-alkanes is proportional to nth power of oxygen mole fraction, [O2]n, with n = 1.5–2. Furthermore, by accounting for thermal and mass transport effects using a mass-weighted enthalpy, a cool flame radical index is proposed to evaluate the role of cool flame reactivity on the extinction limits of large n-alkanes. We find that the radical index of large n-alkanes increases with the increase of carbon length of the fuel molecules, which is consistent with stronger low-temperature reactivity.