A numerical study on NOx formation in laminar counterflow CH4/air triple flames

Formation of NO_x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO_x, compared to the corresponding premixed flames. A triple flame produces more NO and NO₂ than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N₂O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO₂ formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N₂O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.     

Effects of ammonia substitution on combustion stability limits and NOx emissions of premixed hydrogen–air flames

The combustion stability limits and nitrogen oxide (NO_x) emissions of burner-stabilized premixed flames of ammonia (NH₃)-substituted hydrogen (H₂)–air mixtures at normal temperature and pressure are studied to evaluate the potential of partial NH₃ substitution to improve the safety of H₂ use. The effects of NH₃ substitution, nitrogen (N₂) coflow and mixture injection velocity on the stability limits and NO_x emissions of NH₃–H₂–air flames are experimentally determined. Results show a reduction of stability limits with NH₃ substitution and coflow, supporting the potential of NH₃ as a carbon-free, green additive in H₂–air flames and indicating a different tendency from that for no coflow condition. The NO_x emission index is almost constant even with enhanced NH₃ substitution, though the absolute value of NO_x emissions increases in general. At fuel-rich conditions, the NO_x emission index decreases with increasing mixture injection velocity and the existence of coflow. The thermal deNO_x process in the post-flame region is involved in reducing NO_x emissions for the fuel-rich flames.     

An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NOx and PAH species

An experimental and numerical investigation of counterflow prevaporized partially premixed n-heptane flames is reported. The major objective is to provide well-resolved experimental data regarding the detailed structure and emission characteristics of these flames, including profiles of C₁-C₆, and aromatic species (benzene and toluene) that play an important role in soot formation. n-Heptane is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems. A counterflow geometry is employed, since it provides a nearly one-dimensional flat flame that facilitates both detailed measurements and simulations using comprehensive chemistry and transport models. The measurements are compared with predictions using a detailed n-heptane oxidation mechanism that includes the chemistry of NO_x and PAH formation. The reaction mechanism was synergistically improved using pathway analysis and measured benzene profiles and then used to characterize the effects of partial premixing and strain rate on the flame structure and the production of NO_x and soot precursors. Measurements and predictions exhibit excellent agreement for temperature and major species profiles (N₂, O₂, n-C₇H₁₆, CO₂, CO, H₂), and reasonably good agreement for intermediate (CH₄, C₂H₄, C₂H₂, C₃H_x) and higher hydrocarbon species (C₄H₈, C₄H₆, C₄H₄, C₄H₂, C₅H₁₀, C₆H₁₂) and aromatic species (toluene and benzene). Both the measurements and predictions also indicate the existence of two partially premixed regimes; a double flame regime for ?<5.0, characterized by spatially separated rich premixed and nonpremixed reaction zones, and a merged flame regime for ?>5.0. The NO_x and soot precursor emissions exhibit strong dependence on partial premixing and strain rate in the first regime and relatively weak dependence in the second regime. At higher levels of partial premixing, NO_x emission is increased due to increased residence time and higher peak temperature. In contrast, the emissions of acetylene and PAH species are reduced by partial premixing because their peak locations move away from the stagnation plane, resulting in lower residence time, and the increased amount of oxygen in the system drives the reactions to the oxidation pathways. The effects of partial premixing and strain rate on the production of PAH species become progressively stronger as the number of aromatic rings increases.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Swirling distributed combustion for clean energy conversion in gas turbine applications

Distributed combustion provides significant performance improvement of gas turbine combustors. Key features of distributed combustion includes uniform thermal field in the entire combustion chamber, thus avoiding hot-spot regions that promote NO_x emissions (from thermal NO_x) and significantly improved pattern factor. Rapid mixing between the injected fuel and hot oxidizer has been carefully explored for spontaneous ignition of the mixture to achieve distributed combustion reactions. Distributed reactions can be achieved in premixed, partially premixed or non-premixed modes of combustor operation with sufficient entrainment of hot and active species present in the flame and their rapid turbulent mixing with the reactants. Distributed combustion with swirl is investigated here for our quest to explore the beneficial aspects of such flows on clean combustion in simulated gas turbine combustion conditions. The goal is to develop high intensity combustor with ultra low emissions of NO and CO, and much improved pattern factor. Experimental results are reported from a cylindrical geometry combustor with different modes of fuel injection and gas exit stream location in the combustor. In all the configurations, air was injected tangentially to impart swirl to the flow inside the combustor. Ultra-low NO_x emissions were found for both the premixed and non-premixed combustion modes for the geometries investigated here. Swirling flow configuration, wherein the product gas exits axially resulted in characteristics closest to premixed combustion mode. Change in fuel injection location resulted in changing the combustion characteristics from traditional diffusion mode to distributed combustion regime. Results showed very low levels of NO (∼3 PPM) and CO (∼70 PPM) emissions even at rather high equivalence ratio of 0.7 at a high heat release intensity of 36 MW/m³-atm with non-premixed mode of combustion. Results are also reported on lean stability limit and OH^* chemiluminescence under both premixed and non-premixed conditions for determining the extent of distribution combustion conditions.     

NOx emission characteristics of partially premixed laminar flames of H2/CO/CO2 mixtures

NO_x emission indices were experimentally measured for partially premixed laminar flames of five different H₂/CO/CO₂ fuels over a wide range of equivalence ratios. Through those fuels, the effects of H₂/CO ratio and CO₂ concentration on NO_x emissions, flame appearance, visible flame height and flame temperature are presented. EINO_x values increase when 1.0 ≤ Φ ≤ 1.6, then remain near the highest value, before decreasing slowly when 3.85 ≤ Φ ≤ ∞. The increase of the CO₂ concentration reduces the EINO_x for the whole range of equivalence ratios, while the increase in the H₂/CO ratio reduces the EINO_x when Φ ≤ 2.0 and is inconsequential for richer mixtures. The variation in flame temperatures approximates EINO_x trends. The variation of flame color from blue to orange when the H₂/CO ratio is increased might be explained by higher CO levels in by-product combustion.     

Combustion stability limits and NOx emissions of nonpremixed ammonia-substituted hydrogen–air flames

The combustion stability (extinction) limits and nitrogen oxide (NO_x) emissions of nonpremixed ammonia (NH₃)–hydrogen (H₂)–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH₃ substitution for improving the safety of H₂ use and to provide a database for the nonpremixed NH₃-substituted H₂–air flames. Considering coflow nonpremixed NH₃–H₂–air flames for a wide range of fuel and coflow air injection velocities (V_fuel and V_coflow) and the extent of NH₃ substitution, the effects of NH₃ substitution on the stability limits and NO_x emissions of the NH₃–H₂–air flames are experimentally determined, while the nonpremixed NH₃–H₂–air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NO_x emission index for enhanced NH₃ substitution, supporting the potential of NH₃ as an effective, carbon-free additive in nonpremixed H₂–air flames. With increasing V_coflow the NO_x emission index decreases, while with increasing V_fuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given V_coflow/V_fuel the flame length increases with enhanced NH₃ substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH₃ is consumed more upstream than H₂ due to the difference between their diffusivities in air.     

Formation of soot and nitrogen oxides in unsteady counterflow diffusion flames

The formation of pollutant species in turbulent diffusion flames is strongly affected by turbulence/chemistry interactions. Unsteady counterflow diffusion flames can be conveniently used to address the unsteady effects of hydrodynamics on the pollutant chemistry, because they exhibit a larger range of combustion conditions than those observed in steady flames.In this paper, unsteady effects on the formation of soot (and its main precursors) and nitrogen oxides (NO_x) are investigated by imposing harmonic oscillations on the strain rate of several counterflow diffusion flames fed with propane. Numerical results confirm that the dynamic response of each species is strongly affected by the strain rate oscillations and the characteristic time governing its chemistry. At low frequencies of imposed oscillations the soot and NO_x profiles show strong deviations from the steady-state profile. At large frequencies a decoupling between the concentration and the velocity field is evident. In particular, the formation of soot and NO_x is found less sensitive to velocity fluctuations for flames with large initial strain rate. The significant increase of soot and NO_x concentrations in unsteady conditions appears to be a function of both forcing frequency and flame global strain rate. Moreover, the cut-off frequency, defined as the minimum frequency above which the strain rate oscillations have negligible effects on the formation of each species, was found to be strongly dependent on the chemical characteristic time and the flame global strain rate, but only marginally affected by the amplitude of imposed oscillations.     

Modeling fuel NOx formation from combustion of biomass-derived producer gas in a large-scale burner

This study investigates the characteristics of fuel NO_x formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NO_x emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood + 13% DDGS (dried distiller grain soluble) gas and wood + 40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NO_x emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NO_x is very sensitive to the ammonia content in the producer gas. Results show that although NO–NO₂ interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NO_x reduction. The results indicate that NO_x can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NO_x burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NO_x is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.     

A numerical investigation on NOX formation in counterflow n-heptane triple flames

The formation of NO_X in counterflow n-heptane/air triple flames was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results indicate that a triple flame produces more NO and NO₂ than the corresponding premixed flames due to not only the appearance of the diffusion flame but also the interaction between different flame branches. The relative contributions of different routes to NO formation in the premixed flame branches change with the variation of the equivalence ratio, but the thermal mechanism always dominates in the diffusion flame branch. The interaction between flame branches is enhanced with the decrease of the distance between them. Both heat and radical exchange between flame branches contribute to the interaction. A new feature that does not exist in methane/air triple flame was observed in n-heptane/air triple flames, i.e. when the rich mixture equivalence ratio is higher, there are two peaks of CH concentration on the rich side of the diffusion flame branch, which leads to that some NO is formed beside the diffusion flame branch by the prompt route.     

Extinction of laminar partially premixed flames

Flame extinction represents one of the classical phenomena in combustion science. It is important to a variety of combustion systems in transportation and power generation applications. Flame extinguishment studies are also motivated from the consideration of fire safety and suppression. Such studies have generally considered non-premixed and premixed flames, although fires can often originate in a partially premixed mode, i.e., fuel and oxidizer are partially premixed as they are transported to the reaction zone. Several recent investigations have considered this scenario and focused on the extinction of partially premixed flames (PPFs). Such flames have been described as hybrid flames possessing characteristics of both premixed and non-premixed flames. This paper provides a review of studies dealing with the extinction of PPFs, which represent a broad family of flames, including double, triple (tribrachial), and edge flames. Theoretical, numerical and experimental studies dealing with the extinction of such flames in coflow and counterflow configurations are discussed. Since these flames contain both premixed and non-premixed burning zones, a brief review of the dilution-induced extinction of premixed and non-premixed flames is also provided. For the coflow configuration, processes associated with flame liftoff and blowout are described. Since lifted non-premixed jet flames often contain a partially premixed or an edge-flame structure prior to blowout, the review also considers such flames. While the perspective of this review is broad focusing on the fundamental aspects of flame extinction and blowout, results mostly consider flame extinction caused by the addition of a flame suppressant, with relevance to fire suppression on earth and in space environment. With respect to the latter, the effect of gravity on the extinction of PPFs is discussed. Future research needs are identified.     

Numerical study on NOx/CO emissions in the diffusion flames of high-temperature off-gas of steelmaking converter

The combustion of high-temperature off-gas of steelmaking converter with periodical change of temperature and CO concentration always leads to CO and NO_x over-standard emissions. In the paper, high-temperature off-gas combustion is simulated by adopting counterflow diffusion flame model, and some influencing factors of CO and NO_x emissions are investigated by adopting a detailed chemistry GRI 3.0 mechanism. The emission index of NO_x (EINO_x) decreases 1.7–4.6% when air stoichiometric ratio (SR) increase from 0.6 to 1.4, and it dramatically increases with off-gas temperature at a given SR when the off-gas temperature is above 1500 K. High-concentration CO in off-gas can result in high NO_x emissions, and NO_x levels increase dramatically with CO concentration when off-gas temperature is above 1700 K. Both SR and off-gas temperature are important for the increase of CO burnout index (BI_CO) when SR is less than 1.0, but BI_CO increase about 1% when off-gas temperature increases from 1100 K to 1900 K at SR > 1.0. BI_CO increases with CO concentration in off-gas, and the influence of off-gas temperature on BI_CO is marginal. BI_CO increases with the relative humidity (RH) in air supplied, but it increases about 0.5% when RH is larger than 30%.     

Combustion characteristics of a lean premixed LPG–air combustor

Fuel/air mixing effects in a premixer have been examined to investigate the combustion characteristics, such as the emission of NO_x and CO, under simulated lean premixed gas turbine combustor conditions at normal and elevated pressures of up to 3.5 bar with air preheat temperature of 450 K. The results obtained have been compared with a diffusion flame type gas turbine combustor for emission characteristics. The results show that the NO_x emission is profoundly affected by the mixing between fuel and air in the combustor. NO_x emission is lowered by supplying uniform fuel/air gas mixture to the combustor and the NO_x emission reduces with decrease in residence time of the hot gases in the combustor. The NO_x emission level of the lean premixed combustor is a strong function of equivalence ratio and the dependency is smaller for a traditional diffusion flame combustor under the examined experimental conditions. Furthermore, the recirculation flow, affected by dome angle of combustor, reduces the high temperature reaction zone or hot spot in the combustor, thus reducing the NO_x emission levels.     

NOx emissions in n-heptane/air partially premixed flames

NO_x emissions in n-heptane/air partially premixed flames (PPFs) in a counter-flow configuration have been investigated. The flame is computed using a detailed mechanism that combines the Held’s mechanism for n-heptane and the Li and Williams’ mechanism for NO_x. The combined mechanism contains 54 species and 327 reactions. Based on a detailed analysis, dominant mechanisms responsible for NOx formation and destruction in PPFs are found to be thermal, prompt, and reburn mechanisms. The dominant reactions associated with these mechanisms are also identified. The effects of strain rate (a_s) and equivalence ratio (φ) on NO_x emissions are characterized for conditions in which the flame contains two spatially separated reaction zones; a rich premixed zone on the fuel side and a non-premixed zone on the air side. For most conditions, except for relatively high level of partial premixing, the NO formation rate in the non-premixed zone is significantly higher than that in the rich premixed zone. Within the rich premixed zone, the contribution of thermal NO to total NO_x is higher than that of prompt NO, while in the non-premixed zone, the prompt NO is the major contributor. The behavior is related to the transport of acetylene from the rich premixed to the non-premixed zone, and higher concentrations of CH, O, and OH radicals in the latter zone. A notable result in this context is that the existence of CH does not automatically imply that prompt NO will form. The existence of O and OH is also necessary, in addition to CH, to form prompt NO. The relative contributions of thermal and prompt mechanisms to total NO_x are generally insensitive to variations in a_s, but show strong sensitivity to variations in φ. There is a NO_x destruction region sandwiched between the rich premixed and the non-premixed reaction zones. The NO_x destruction occurs mainly through the reburn mechanism. The NO_x emission index (EINOx) is computed as a function of φ and a_s. These results are qualitatively in accord with previous numerical and experimental results for methane-air PPFs.     

Emission of impinging swirling and non-swirling inverse diffusion flames

The overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs) was evaluated quantitatively by the ‘hood’ method. The results of in-flame volumetric concentrations of CO and NO_x and overall pollutants emission of CO and NO_x in terms of emission index were reported. The in-flame volumetric concentrations of CO and NO_x were measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NO_x concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINO_x increases as the nozzle-to-plate distance (H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and non-swirling IDFs and the comparison shows that the swirling IDF emits less NO_x and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NO_x and higher level of CO.     

Investigation of NOx conversion characteristics in a porous medium

The conversion of nitric oxide (using CNG/air as fuel/oxidizer) inside a porous medium is investigated in this study. Unlike freely propagating flames, porous burners provide a solid medium that facilitates heat exchange with the gaseous phase. The heat exchange allows the stabilization of a variety of fuel mixtures from lean to rich and with a variety of calorific values. In addition, it allows the control of the reaction zone temperature and thus the control of pollutant formation while maintaining flame stability. An experimental porous burner was designed and manufactured for this purpose. The effects of equivalence ratio and flow velocity on the flame stabilization, NO_x and TFN (total fixed nitrogen) conversion ratios, and temperature profiles along the burner are investigated. In addition, numerical calculations using the PLUG flow simulator model and the GRI 3.0 kinetic mechanism reveals the key reactions which control the conversion efficiency. It was found that under slightly fuel-rich conditions (φ?1.3) NO_x mostly converts to N₂ with a maximum conversion ratio of 65%, while for higher equivalence ratios (φ>1.3) a large proportion of NO_x converts to NH₃. Results from experiments and numerical modeling showed that the temperature profile along the burner has significant effects on the NO_x and TFN conversion ratios. It was also found that temperatures between 1000 and 1500 K are most desirable for NO_x and TFN conversion in the porous burner. Analysis of the chemical paths for the low- and high-equivalence-ratio cases showed that the formation of nitrogen-containing species under very rich conditions (φ>1.3) is due to the increased importance of the HCNO path as compared to the HNO path. The latter is the dominant path at low equivalence ratios (φ?1.3) and leads to the formation of N₂. The NO concentration in the initial mixture was found to improve the conversion by up to 20% at low equivalence ratios (φ?1.3) and to have negligible effect at higher equivalence ratios.     

Impact of H2 addition on flame stability and pollutant emissions for an atmospheric kerosene/air swirled flame of laboratory scaled gas turbine

The purposes of this study are to compare the stability domains and the pollutant emissions when combustion occurs with and without addition of H₂ to a kerosene (Jet A1)/air premixed prevaporised mixture injected in a lean gas turbine combustor. Chemiluminescence of CH*, pollutant emissions (NO_x and CO) and pressure fluctuations data are simultaneously collected in order to determine the effects of H₂ addition on the stability of the combustion and on the flame structure for an inlet temperature of 473 K, atmospheric pressure and for a large range of equivalence ratio (from 0.3 to 1). Addition of hydrogen enables keeping stable combustion conditions when, for the same kerosene mass flow, the flame becomes lifted and very unstable. As for pollutant emissions, results show that the equivalence ratio is the key parameter to control NO_x emission even in the situation where the combustion power is increased due to H₂ addition. As H₂ addition strongly increases the flammability limits and the combustion stability domain, stable combustion can occur at leaner equivalence ratio and then decreases CO and NO_x emissions. This is an important fact since no substitution effect takes place in the reduction of NO_x and CO emissions. Study at constant combustion power and equivalence ratio by adjusting hydrogen and kerosene mass flow shows again a decrease in the pollutant emissions. Hydrogen injection in power generation systems using combustion seems to be a promising way in combustion research since due to the combined effects of enlarging combustion stability domain and reducing NO_x emissions by substituting kerosene to the benefit of H₂.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Pilot impact on turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H₂/CH₄: 50/50 in vol%) fuels was investigated in a gas turbine model combustor under atmospheric conditions. The burner assembly was designed to mimic the geometry of an industrial burner, the Siemens DLE Burner, in which a concentric annular ring equipped with pilot flame burners is implemented in the dome of the combustor. Two pilot burner configurations have been investigated: a non-premixed and a partially premixed pilot arrangement. The performance of the pilot burners was evaluated for varying Reynolds number (Re) and H₂ enrichment. High-speed OH1 chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH₂O) were used for evaluating the dynamics and structures of the flames for different conditions. Furthermore, emission measurements were carried out to determine the influence of hydrogen dilution on the NO_x and CO emission levels. The main findings are (a) the effect of the pilot flame is sensitive to the Reynolds number of the main flame and the type of the pilot flame, (b) the stability range becomes narrower with increasing hydrogen ratio, due to the tendency to flashback, (c) non-premixed pilot flames lower the NO_x and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NO_x and CO emissions become significantly lower with increasing hydrogen ratio.     

Experimental and kinetic modeling study of pyrolysis and oxidation of n-decane

The pyrolysis of n-decane was investigated in a flow reactor at 5, 30, 150 and 760 Torr, and the oxidation of n-decane at equivalence ratios of 0.7, 1.0 and 1.8 was studied in laminar premixed flames at 30 Torr. In both experiments, synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to identify combustion species and measure their mole fraction profiles. A new detailed kinetic model of n-decane with 234 species and 1452 reactions was developed for applications in intermediate and high temperature regions, and was validated against the experimental results in the present work. The model was also validated against previous experimental data on n-decane combustion, including species profiles in pyrolysis and oxidation in high pressure shock tube and atmospheric pressure flow reactor, jet stirred reactor oxidation, atmospheric pressure laminar premixed flame, counterflow diffusion flame and global combustion parameters such as laminar flame speeds and ignition delay times. In general, the performance of the present model in reproducing these experimental data is reasonably good. Sensitivity analysis and rate of production analysis were conducted to understand the decomposition processes of n-decane.