Lawrence Berkeley National Laboratory report LBNL-54187 Effects of Mixing on Ammonia Oxidation in Combustion Environments at Intermediate Temperatures (original) (raw)
Related papers
Effects of mixing on ammonia oxidation in combustion environments at intermediate temperatures
Proceedings of the Combustion Institute, 2005
Ammonia often occurs in combustion gases as a product of fuel-nitrogen. Since either NO or N 2 may predominate as the product of ammonia oxidation, there is considerable interest in understanding the factors responsible for selecting the final products. In flames the selectivity is known to depend on whether the reaction zones are premixed or non-premixed. This paper reports on a combined experimental and modeling investigation of ammonia chemistry in a hot combustion environment that is below flame temperatures, such as in post combustion gases. Experiments that mix highly diluted ammonia-methane and oxygen-water streams are interpreted in terms of a plug-flow model, a simplified mixing reactor model, and a two-dimensional direct numerical simulation. The study finds that the final products of ammonia oxidation remain sensitive to mixing even at temperatures below those of self-sustaining flames. At low temperatures ammonia oxidation occurs in a premixed reaction zone, but at sufficiently high temperatures a nonpremixed reaction zone may develop that produces significantly less NO than the equivalent premixed system. A direct numerical simulation is required to predict the behavior over the full range of conditions investigated experimentally, while a simplified mixing reactor model captures the essential features as long as the radial gradients are not too steep.
Combustion hazard of mixing ammonia with nitric oxide
Journal of Loss Prevention in the Process Industries, 2003
Premixed ammonia/nitric oxide flame was simulated using the Lindstedt 1994 and Miller-Bowman 1989 reaction mechanisms in CHEMKIN. The predicted laminar burning velocities compared well with limited measured values in the literature. The effects of unburnt mixture temperature and pressure on laminar burning velocity, flammability limits, adiabatic flame temperature and species profiles were studied. The unburnt mixture temperature had a positive impact on both the laminar burning velocity and the adiabatic flame temperature, and it extended the ammonia-rich flammability limit. The pressure had a marginally negative influence on the laminar burning velocity, while it had a slightly positive effect on the adiabatic flame temperature.
Counterflow flame extinction of ammonia and its blends with hydrogen and C1-C3 hydrocarbons
Applications in Energy and Combustion Science
Ammonia as a fuel offers the potential to avoid carbon emissions, but its combustion is hindered by low reactivity. Here, the extinction limits of NH 3 and NH 3 plus reactivity enhancers were measured in the counterflow laminar non-premixed flames. A stable NH 3-N 2 flame was established with an oxygen-enriched oxidizer stream, and when the fuel was blended with CH 4 , C 2 H 6 , C 3 H 8 , and H 2. For blended mixtures, results showed that CH 4 has the least potential to enhance the stability of NH 3 flames compared to the other additives. The extinction limits of C 2 H 6 and C 3 H 8 blended NH 3 flames are nearly identical. At low percentage addition, H 2-blended flames extinguish earlier than those blended with C 1-C 3 hydrocarbons, but this trend is reversed at higher H 2 blends. Experimental conditions were simulated using Okafor et al. 2018 model and an extended Zhang et al 2021 model developed here. The models captured the measured trends, including the crossover between NH 3-H 2 and NH 3-C 2 /C 3 hydrocarbon fuels. Quantitatively, both models under-predicted the extinction limits of NH 3-N 2 /enriched oxidizer flame. Better quantitative agreement is observed for the blended fuels using the model developed here. Discrepancies have been observed in the reported rates for reactions involving HNO (+OH, H), and if addressed, could improve models' capability in predicting extinction behavior in non-premixed flames. Numerical analyses were carried out to understand the kinetic coupling between NH 3 and H 2 /C 2-C 3 in counter-flow flames. Extinction limits of NH 3-C 2-C 3 /H 2 flames are shown to be affected by H abstraction and NH 3 related chain termination reactions, heat producing reactions, and chain branching reactions. It has also been observed that at high blending ratios, C 2 H 6 /C 3 H 8 addition in NH 3 flames reduced the peak H and OH concentration via recombination and termination reactions, which compete with branching pathways. H 2-blended flames are mostly influenced by reactions producing active radicals.
Combustion Characteristics of Ammonia as a Promising Renewable Fuel
Proceedings of the World Congress on Momentum, Heat and Mass Transfer, 2016
Introduced as a renewable energy source in many studies, ammonia (NH3) is known as a potential fuel to be combusted in power engines. With its high hydrogen density and already existing infrastructure, it is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. A comprehensive numerical study is accomplished focusing on the major combustion characteristics of ammonia and ammonia-hydrogen flames in a wide range of conditions.
2020
Ammonia is a promising sustainable fuel, however, its low reactivity creates challenges in industrial applications. In this study, ammonia/methane mixtures were considered for premixed and non-premixed counterflow flames. The extinction stretch rate was measured over a wide range of ammonia/methane mixing ratios and compared to 1D numerical results from four different mechanisms. Additionally, for counterflow premixed twin flames, quantitative analysis based on the comparison of experimental and numerical FWHM of OH and NO profiles was performed. Results showed that in premixed flames, all the mechanisms investigated were inadequate for predicting the extinction stretch rate, specifically for lean flames. In non-premixed flames, Okafor's mechanism was accurately predicting the extinction stretch rate. For the FWHM analysis, the numerical mechanisms overpredicted both OH and NO apparition in the flame, except for Tian's mechanism which underpredicted OH apparition. GRI Mech 3...
Science and technology of ammonia combustion
Proceedings of the Combustion Institute, 2018
This paper focuses on the potential use of ammonia as a carbon-free fuel, and covers recent advances in the development of ammonia combustion technology and its underlying chemistry. Fulfilling the COP21 Paris Agreement requires the de-carbonization of energy generation, through utilization of carbon-neutral and overall carbon-free fuels produced from renewable sources. Hydrogen is one of such fuels, which is a potential energy carrier for reducing greenhouse-gas emissions. However, its shipment for long distances and storage for long times present challenges. Ammonia on the other hand, comprises 17.8% of hydrogen by mass and can be produced from renewable hydrogen and nitrogen separated from air. Further more, ther mal properties of ammonia are similar to those of propane in terms of boiling temperature and condensation pressure, making it attractive as a hydrogen and energy carrier. Ammonia has been produced and utilized for the past 100 years as a fertilizer, chemical raw material, and refrigerant. Ammonia can be used as a fuel but there are several challenges in ammonia combustion, such as low flammability, high NOx emission, and low radiation intensity. Overcoming these challenges requires further research into ammonia flame dynamics and chemistry. This paper discusses recent successful applications of ammonia fuel, in gas turbines, co-fired with pulverize coal, and in industrial furnaces. These applications have been implemented under the Japanese 'Cross-ministerial Strategic Innovation Promotion Program (SIP): Energy Carriers'. In addition, fundamental aspects of ammonia combustion are discussed including characteristics of laminar premixed flames, counterflow twin-flames, and turbulent premixed flames stabilized by a nozzle burner at high pressure. Furthermore, this paper discusses details of the chemistry of ammonia combustion related to NOx production, processes for reducing NOx, and validation of several ammonia oxidation kinetics models. Finally, LES results for a gas-turbine-like swirl-burner are presented, for the purpose of developing low-NOx single-fuelled ammonia gas turbine combustors.
International Journal of Hydrogen Energy, 2018
To achieve comprehensive prediction of ammonia combustion in terms of flame speed and ignition delay time, an improved mechanism of ammonia oxidation was proposed in this work. The present model (UT-LCS) was based on a previous work [Song et al., 2016] and improved by relevant elementary reactions including NH 2 , HNO, and N 2 H 2. The model clearly explained reported values of laminar flame speed and ignition delay time in wide ranges of equivalence ratio and pressure. This suggests that NH 2 , HNO, and N 2 H 2 reactivities play a key role to improve the reaction mechanism of ammonia oxidation in the present model. The model was also applied to demonstrate NH 3 /H 2 /air combustion. The present model also appropriately predicted the laminar flame speed of NH 3 /H 2 /air combustion as a function of equivalence ratio. Using the model, we discussed the reduction of NO concentration downstream and H 2 formation via NH 3 decomposition in NH 3 /H 2 fuelrich combustion. The results provide suggestions for effective combustion of NH 3 for future applications.
Ammonia conversion and NOx formation in laminar coflowing nonpremixed methane-air flames
Combustion and Flame, 2002
This paper reports on a combined experimental and modeling investigation of NO x formation in nitrogen-diluted laminar methane diffusion flames seeded with ammonia. The methane-ammonia mixture is a surrogate for biomass fuels which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measure the concentration of stable species in the exhaust gas, including NO, O 2 , CO, and CO 2. The computations evolve a two-dimensional low Mach number model using a solution-adaptive projection algorithm to capture fine-scale features of the flame. The model includes detailed thermodynamics and chemical kinetics, differential diffusion, buoyancy, and radiative losses. The models shows good agreement with the measurements over the full range of experimental NH 3 seeding amounts. As more NH 3 is added, a greater percentage is converted to N 2 rather than to NO. The simulation results are further analyzed to trace the changes in NO formation mechanisms with increasing amounts of ammonia in the fuel.
NO Formation and Autoignition Dynamics during Combustion of H2O-Diluted NH3/H2O2 Mixtures with Air
Energies
NO formation, which is one of the main disadvantages of ammonia combustion, was studied during the isochoric, adiabatic autoignition of ammonia/air mixtures using the algorithm of Computational Singular Perturbation (CSP). The chemical reactions supporting the action of the mode relating the most to NO were shown to be essentially the ones of the extended Zeldovich mechanism, thus indicating that NO formation is mainly thermal and not due to fuel-bound nitrogen. Because of this, addition of water vapor reduced NO formation, because of its action as a thermal buffer, but increased ignition delay, thus exacerbating the second important caveat of ammonia combustion, which is unrealistically long ignition delay. However, it was also shown that further addition of just 2% molar of H2O2 does not only reduce the ignition delay by a factor of 30, but also reverses the way water vapor affects ignition delay. Specifically, in the ternary mixture NH3/H2O/H2O2, addition of water vapor does not ...