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Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO 2 and H 2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid. • The premixed formic acid-methane-air flames are stabilized for equivalence ratios of 0.5–0.8. • The addition of the formic acid up to mole fraction of 0.6 increases both the flashback and blowout limit. • Increasing the Reynolds number does not affect blow out limits. • The swirl number has a non-monotonous effect on the flashback and lean blowout limits. • The NO and CO emission of the lean formic acid-methane-air flames are low (<20 ppmvd). [ABSTRACT FROM AUTHOR]

Experimental and numerical studies were performed to examine the stability of inverse diffusion flames (IDFs) with the focus on the boundary wall effects. A regime diagram for flame stability was obtained based on the visual characteristics of flames and verified by the simulated flow fields. The boundary wall effects were identified and investigated by simulating the IDFs with different outer burner diameters. It was found that the wall-bounded induced shear flows either reduce the flow instability by vorticity diffusion or enhance it by vorticity convection. Based on the experimental and numerical observations, the mechanism of the boundary wall effects on the stability of buoyant IDFs was explained and further extended to general diffusion flames. [ABSTRACT FROM AUTHOR]

Experiment and simulation were performed to investigate buoyant methane/air inverse diffusion flames, with emphasis on the near-field flow dynamics under non-reacting and reacting conditions. In the non-reacting flow, the initial shear flow and the buoyancy effect induce opposite-direction vortices, which interact with each other and cause flow instability similar to the mechanism forming the von Karman vortex street. The instability is greatly intensified at around unity Richardson number, when the two vortices are comparably strong. In the reacting flows, the density gradient is reversed due to chemical heat release and so is the buoyancy-induced vortex that has the same direction with the vortex of the initial shear flow. As a result, the buoyancy-induced vorticity generation would facilitate the growth of the initial shear layer, thus the near-field flow remains stable. However, the growing shear flow would eventually lead to the development of the Kelvin–Helmholtz instability in the far field. [ABSTRACT FROM AUTHOR]

Ammonia has the potential of being a future fuel for carbon-free combustion to reduce greenhouse gases (GHG) emission. The heat release rate (HRR) information of ammonia flames is desired if the ammonia fuel is applied in real combustion devices, but direct measurement of HRR is almost impossible. In the present work, the HRR markers of ammonia-methane-air flames were investigated through the simulation of one-dimensional freely propagating flames (FPF) with a validated chemistry mechanism. Various HRR markers were proposed. The performance of the target HRR markers was tested with the defined criteria. It was found that the profiles of NH and HCO agree well with the HRR profiles, but only NH is a suitable HRR marker because the mole fraction of HCO could be too low for laser induced fluorescence (LIF) measurement. The products of two species, such as [OH][CH 2 O] and [NO][NH 2 ], were then proposed to circumvent the low mole fraction issues. Product [OH][CH 2 O], which was usually used as HRR marker in hydrocarbon flames, was found to have the best performance in identifying the HRR location and be the only one that has a positive correlation with the amount of HRR among the proposed products. Moreover, the performance of [OH][CH 2 O] is valid over a wide range of conditions, including the very high ammonia fuel fraction (X NH3 = 95%), without suffering from the low mole fraction issues. The observations were further analyzed by looking into the reaction pathways and examining the reactions that contributes the largest amount to the total HRR. The present findings provide a meaningful reference for future indirect measurement of HRR in ammonia-methane-air flames. [ABSTRACT FROM AUTHOR]

This paper reports on the chemiluminescence footprint of premixed ammonia-methane-air flames. The chemiluminescence spectrum of laminar twin-flames stabilized with a counterflow burner were measured between 200 and 457 nm for large ranges of equivalence ratio (0.60 ≤ ϕ ≤ 1.30), ammonia fraction (pure methane to pure ammonia), and strain rate (55/s ≤ a ≤ 300/s). Relevant excited radicals were identified, namely, NO*, OH*, NH*, CN*, CH*, and CO 2 * and evolutions of their chemiluminescence intensity were analyzed as a function of equivalence ratio, ammonia fraction, and strain rate. These measurements produced an unprecedented database on the chemiluminescence of ammonia-methane-air flames, which could be used in the future for model validation. A total of 15 ratios of chemiluminescence intensity were also considered and 5 ratios showing promise for the development of chemiluminescence-based flame sensors were identified. The CN*/OH* ratio is a potential surrogate for equivalence ratio even if the ammonia fuel fraction in the fuel blend is not known accurately – as long as it exceeds 0.3 by volume. The CN*/NO* ratio is another possible surrogate for equivalence ratio if the ammonia fraction in the fuel blend is below 0.5. The OH*/CH* ratio, often used to sense equivalence ratio in hydrocarbons flames, is not recommended for ammonia-methane flames. The NH*/CN* ratio is a potential surrogate for the ammonia fraction in the fuel blend if equivalence ratio is larger than ϕ = 0.7 and if the ammonia fraction in the fuel blend is below 0.4. Other ratios may be combined to provide a simultaneous measure of equivalence ratio and ammonia fraction in the fuel blend with an extended range of validity, for example NH*/OH* and CN*/NH*. [ABSTRACT FROM AUTHOR]

Hydrogen is a promising carbon-free fuel for power generation in gas turbines. However, this raises some challenges associated with the storage and distribution of pure hydrogen. Storing hydrogen chemically in the form of ammonia is a safe and efficient alternative. However, ammonia as a fuel features a low chemical reactivity compared to hydrogen and natural gas and, as a consequence, stabilizing turbulent ammonia-air flames is challenging. Offsetting this low reactivity by enriching ammonia with some amount of hydrogen, which is much more reactive, is a promising strategy. In this study, the stability limits of technically-premixed ammonia-hydrogen-air flames are measured in a laboratory-scale swirl combustor for a wide range of ammonia fractions in the ammonia-hydrogen fuel blend. Results are compared to that obtained in the same combustor for reference methane-hydrogen-air mixtures. Data show that increasing the ammonia fraction in the fuel blend promotes lean blowout but reduces the flames' propensity to flashback. The latter effect is even more pronounced if the volume fraction of ammonia in the fuel blend exceeds 0.7. In that case, increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blowout occurs instead, yielding a much wider range of stable equivalence ratios. This study also reports exhaust NO mole fractions, measured for large ranges of equivalence ratio and ammonia fraction in the fuel blend. Regardless of the ammonia fraction, data show that competitively low NO emissions occur for slightly rich equivalence ratios of φ ≥ 1.05, which is consistent with earlier studies. Stable flames and good NO performance are also found for very lean ammonia-hydrogen-air mixtures with φ ≤ 0.50, demonstrating the strong potential of fueling gas turbines with ammonia-hydrogen blends. • Ammonia-hydrogen-air flames feature wider stability limits than ammonia-air or hydrogen-air flames. • Nitrogen addition to ammonia-hydrogen fuel blends does not significantly alter stability limits. • Pure ammonia-air flames feature lower NO emissions than ammonia-hydrogen-air flames. • Good NO performance is found for "very" lean ammonia-hydrogen-air flames. [ABSTRACT FROM AUTHOR]

This work provides a kinetic and experimental investigation of excited radical OH(2Σ+) (also referred to as OH*) in NH 3 H 2 -air flames. A counterflow burner is used to stabilize laminar diffusion flames over wide ranges of ammonia fraction in the fuel blend (0.2 ≤ x NH3 ≤ 0.8) and strain rate (40 ≤ a ≤ 200/s). Using an intensified camera or a spectrometer coupled to a Cassegrain optical system, spatially resolved and spatially integrated OH(A2Σ+-X2Π) chemiluminescence intensities are measured. These data are used to challenge a kinetic mechanism largely developed from existing literature schemes. Measurements and simulations show that two distinct OH* peaks exist in spatially resolved profiles for intermediate ammonia fractions in the fuel blend. Sensitivity analyses identified that reactions N 2 O+ H =N 2 +OH* and H + O + M =OH*+ M , respectively pertinent to NH 3 and H 2 oxidation routes, are responsible for the formation of OH*. The proposed kinetic mechanism gives a remarkable portrait of the empirical data recorded in diffusion flames as well as in premixed NH 3 H 2 -air flames from the literature. Nevertheless, the contribution of reaction N 2 O+ H =N 2 +OH* in the formation of OH* in the peak closest to the fuel side of diffusion flames is consistently overpredicted. Consequently, the frequency factor of the N 2 O+ H N 2 +OH* reaction is adjusted to A = 1.35 × 1014 cm3mol−1s−1, which significantly improves predictions of spatially integrated and spatially resolved OH* intensities for all the diffusion and premixed flames examined. [ABSTRACT FROM AUTHOR]

• Pure ammonia-air swirl flames can be stabilized for a wide range of equivalence ratios. • Ammonia addition to methane reduces the flames' propensity to flashback. • Ammonia addition to methane widens the stable range of operation. • Good NO performances are only achieved for slightly-rich equivalence ratios. Ammonia is a promising fuel for heat and power generation because it is carbon-free and it can be produced from abundant chemicals and renewable energy sources. However, ammonia-air mixtures feature a low reactivity and stabilizing turbulent ammonia-air flames is challenging. In this study, the stability limits of technically-premixed ammonia-methane-air mixtures are measured for a wide range of ammonia additions in a laboratory-scale swirl burner. Results are compared to that obtained for baseline methane-air mixtures. Data show that increasing the ammonia addition increases the equivalence ratio at the lean blowout limit but also reduces the flames' propensity to flashback. If the volume fraction of ammonia in the fuel blend exceeds a critical value, experiments also show that increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blow-out occurs instead. This significantly widens the range of equivalence ratios yielding stable flames. The critical ammonia volume fraction is a function of the Reynolds number and is x NH3 = 0.42 for Re = 7000 and x NH3 = 0.70 for Re = 3000. Because the ability to stabilize ammonia-methane-air flames is not practically relevant if NOx emissions are too large compared to that of conventional methane-air flames, exhaust NO concentrations are also measured. Consistent with previous studies, data show that, for non-marginal ammonia fuel fractions, good NO performances are not found for lean ammonia-methane-air mixtures and can only be achieved with slightly rich mixtures, which are within measured stability limits. [ABSTRACT FROM AUTHOR]