Combustion of NH₃/CH₄/Air and NH₃/H₂/Air Mixtures in a Porous Burner: Experiments and Kinetic Modeling

Abstract

Ammonia (NH 3 ) is currently in the spotlight because of its potential in serving as an energy resource devoid of carbon. Combustion of NH 3 , however, presents several drawbacks, such as high ignition temperature, low flame speed, and high NO x emissions. Furthermore, although dual-fuel approaches may provide a solution for some of these problems, scarcity of information on the resulting emissions inhibits a large-scale adoption. Therefore, the present work is focused on studying gaseous pollutants, namely, NO x , CO, and unburned NH 3 , emitted from premixed flames of mixtures of NH 3 and either CH 4 or H 2 as combustion enhancers in a porous media burner. An inert alumina−zirconia porous media-based burner fueled with NH 3 /CH 4 /air and NH 3 /H 2 /air mixtures was employed. Temperatures and concentrations of NO x , CO, and NH 3 were measured as a function of the NH 3 molar fraction in the fuel mixture for both type of mixtures. The experimental data were compared to results from simulations performed with recently developed chemical kinetic mechanisms, updated with the latest nitrogen chemistry submechanisms. The experimental results for NH 3 /CH 4 mixtures showed that the NO x emissions peak at a NH 3 molar fraction in the fuel mixture of around 0.5, while for NH 3 /H 2 mixtures, the NO x emissions present maxima at NH 3 molar fractions in the fuel mixture of 0.5 and 0.8. The CO emission data indicated complete combustion of CH 4 , but the presence of unburnt NH 3 in the flue gas reveals its incomplete oxidation for the studied conditions. The kinetic simulations showed similar NO x emission patterns but significantly overpredicted the experimental data in most cases.

Structure and Laminar Flame Speed of an Ammonia/Methane/Air Premixed Flame under Varying Pressure and Equivalence Ratio

“…The authors also indicated that competition for oxygen may promote CO emissions under some conditions. Rocha et al 44 measured NO x and unburned NH 3 emissions in NH 3 /H 2 /air and NH 3 /CH 4 /air flames in a porous media burner, which attested to the effects of the reaction paths indicated in the previous work. 43 Laser-based methods are highly valuable for non-intrusive measurements in combustion to provide data for model validation and have also been applied in studies of ammonia combustion.…”

“…The authors also indicated that competition for oxygen may promote CO emissions under some conditions. Rocha et al measured NO x and unburned NH 3 emissions in NH 3 /H 2 /air and NH 3 /CH 4 /air flames in a porous media burner, which attested to the effects of the reaction paths indicated in the previous work …”

“…A previous study of ammonia/hydrogen and ammonia/methane premixed flames indicated that NO formation is highly sensitive to the reaction HNO + OH = NO + H 2 O, being the largest contributor to NO production in several of their studied flames. 44 This reaction is suppressed at elevated pressure, also promoting the lower NO formation at 3 bar compared to 1 bar. Laminar Flame Speed.…”

“…Investigations were carried out under different molar fractions of NH 3 in the NH 3 and CH 4 mixture (0.2–0.8), equivalence ratios (0.8–1.2), and pressures (1–3 bar). Fuel/air equivalence ratios (ϕ) are calculated considering full conversion of ammonia into H 2 O and N 2 as well as full conversion of CH 4 into H 2 O and CO 2 (see formulation by Rocha et al). In this section, a brief description of the experimental rig is presented, while more details about the rig can be found in refs and .…”

Numerical Analysis on the Evolution of NH2 in Ammonia/hydrogen Swirling Flames and Detailed Sensitivity Analysis under Elevated Conditions

“…Predominantly, the use of hydrogen has been attempted in power devices fueled with ammonia (Khateeb et al 2020;Pacheco et al 2021;Rocha et al 2019;Zhu et al 2021). Further studies considered the use of ammonia/hydrogen blends at concentrations of 60/40, 70/30, 80/20 and 90/10% (vol) (Valera-Medina et al 2018a).…”