A pyrolysis mechanism for ammonia

Probing High-Temperature Amine Chemistry: Is the Reaction NH3 + NH2 ⇄ N2H3 + H2 Important?

The reaction NH₃ + NH₂ ⇄ N₂H₃ + H₂ (R1) has been identified as a key step to explain experimental results for pyrolysis and oxidation of ammonia. However, no direct experimental or theoretical evidence for the reaction has been reported. In the present work, the reaction was studied by ab initio theory and by reinterpretation of experimental data. We could not locate a transition state for R1 occurring as a direct process, but alternative mechanisms yielded an upper bound to k₁ of 1.5 × 10¹³ exp(-58.9 kcal mol^-1/RT) cm³ mol^-1 s^-1 over 1000-2500 K, several orders of magnitude lower than values applied in modeling. Consistent with the theoretical work, re-evaluation of NH₃ pyrolysis data supported a very low value of k₁. However, this finding opens up a novel unresolved issue. Current kinetic models cannot capture the NH₃ oxidation behavior in a number of laminar flow reactor and jet-stirred reactor experiments without adopting an improbably high value for k₁. Important oxidation steps might be underestimated or missing from mechanisms.

Non-premixed combustion and NOX emission characteristics in a micro gas turbine swirl combustor fueled by methane and ammonia at various heat loads

In comparison to methane (CH₄), ammonia (NH₃) is considered a potential carbon-free alternative fuel that can reduce greenhouse gas emissions. But a principal concern is the generation of elevated nitrogen oxide (NO_X) emissions from NH₃ flame. In this study, the detailed reaction mechanisms and thermodynamic data of CH₄ oxidation and NH₃ oxidation were performed using the steady and unsteady flamelet models. After validation of the turbulence model, the combustion and NO_X emission characteristics of CH₄/air and NH₃/air non-premixed flames in a micro gas turbine swirl combustor under a series of identical heat loads were numerically investigated and compared. The present results show that the high-temperature zone of the NH₃/air flame migrates more rapidly toward the outlet of the combustion chamber than that of the CH₄/air flame as the heat load increases. The average NO, N₂O, and NO₂ emission concentrations at all heat loads from NH₃/air flame are respectively 6.12, 161.05 (given the very low N₂O emission concentration from CH₄/air flame), and 2.89 times higher than those from CH₄/air flame. There are correlation trends between some parameters (e.g. characteristic temperature and OH emissions) with the variation of the heat load, and the relevant parameters can be tracked to predict the emission trends after changing the heat load.

Kinetic Study for Plasma Assisted Cracking of NH3: Approaches and Challenges

Ammonia is considered as one of the promising hydrogen carriers toward a sustainable world. Plasma assisted decomposition of NH₃ could provide cost- and energy-effective, low-temperature, on-demand (partial) cracking of NH₃ into H₂. Here, we presented a temperature-dependent plasma-chemical kinetic study to investigate the role of both electron-induced reactions and thermally induced reactions on the decomposition of NH₃. We employed a plasma-chemical kinetic model (KAUSTKin), developed a plasma-chemical reaction mechanism for the numerical analysis, and introduced a temperature-controlled dielectric barrier discharge reactor for the experimental investigation using 1 mol % NH₃ diluted in N₂. As a result, we observed the plasma significantly lowered the cracking temperature and found that the plasma-chemical mechanism should be further improved to better predict the experiment. The commonly used rates for the key NH₃ pyrolysis reaction (NH₃ + M ↔ NH₂ + H + M) significantly overpredicted the recombination rate at temperatures below 600 K. Furthermore, the other identified shortcomings in the available data are (i) thermal hydrazine chemistry, (ii) electron-scattering cross-section data of N_xH_y, (iii) electron-impact dissociation of N₂, and (iv) dissociative quenching of excited states of N₂. We believe that the present study will spark fundamental interest to address these shortcomings and contribute to technical advancements in plasma assisted NH₃ cracking technology.

Fire Safety Evaluation of High-Pressure Ammonia Storage Systems

Ammonia combustion is a promising energy source as a carbon free fuel without greenhouse gas emissions. However, since the auto-ignition temperature is 651 degrees Celsius and the range of flammability limit is not wide compared to other fuels, fundamental studies on ammonia fires have rarely been conducted so far. Therefore, this study aims to numerically estimate fire spread characteristics when ammonia fuel in a high-pressure state leaks to the outside, especially focusing on the flammability limit according to oxygen concentration. Three kinds of reaction mechanism for numerical analysis were adopted to compare the flame structure, flammability limit, and combustion characteristics. Plank-mean absorption coefficients of nitrogen species were taken for the radiation model, in addition to the optically thin model. The effect of radiation heat loss could be identified from the maximum flame temperature trend at a low strain rate. It was confirmed that the pyrolysis of ammonia in the preheated zone results in hydrogen production, and the generated hydrogen contributes to heat release rate in the flame zone. It is found that the contribution of hydrogen would be an important role in the flammability limit of ammonia combustion. Finally, Karlovitz and Peclet numbers showed well the extinction behaviors of ammonia combustion as a result of LOC (Limit Oxygen Concentration) analysis as a function of global strain rate.

Experimental Study of the Pyrolysis of NH3 under Flow Reactor Conditions

The possibility of using ammonia (NH₃), as a fuel and as an energy carrier with low pollutant emissions, can contribute to the transition to a low-carbon economy. To use ammonia as fuel, knowledge about the NH₃ conversion is desired. In particular, the conversion of ammonia under pyrolysis conditions could be determinant in the description of its combustion mechanism. In this work, pyrolysis experiments of ammonia have been performed in both a quartz tubular flow reactor (900-1500 K) and a non-porous alumina tubular flow reactor (900-1800 K) using Ar or N₂ as bath gas. An experimental study of the influence of the reactor material (quartz or alumina), the bulk gas (N₂ or Ar), the ammonia inlet concentration (1000 and 10 000 ppm), and the gas residence time [2060/T (K)-8239/T (K) s] on the pyrolysis process has been performed. After the reaction, the resulting compounds (NH₃, H₂, and N₂) are analyzed in a gas chromatograph/thermal conductivity detector chromatograph and an infrared continuous analyzer. Results show that H₂ and N₂ are the main products of the thermal decomposition of ammonia. Under the conditions of the present work, differences between working in a quartz or non-porous alumina reactor are not significant under pyrolysis conditions for temperatures lower than 1400 K. Neither the bath gas nor the ammonia inlet concentration influence the ammonia conversion values. For a given temperature and under all conditions studied, conversion of ammonia increases with an increasing gas residence time, which results into a narrower temperature window for NH₃ conversion.

Contributions of Experimental Data Obtained in Concentrated Mixtures to Kinetic Studies: Application to Monomethylhydrazine Pyrolysis

Experimental, numerical, and theoretical studies are performed to understand the explosive thermal decomposition of monomethylhydrazine/argon mixtures. Ignition delays of concentrated MMH/Ar mixtures (20-30%) have been measured behind a reflected shock wave around 1000 K and 1 atm. Although several detailed chemical kinetic models have predictive abilities for diluted and highly diluted mixtures, none of them showed predictive for concentrated mixtures. A new kinetic model is proposed, in which numerous rate constants and thermochemical data are reassessed based on theoretical calculations, with the purpose to determine whether, or to what extent, trends derived from diluted or highly diluted MMH/Ar mixtures can explain observations in concentrated MMH mixtures. The present kinetic model is found to predict speciation experimental profiles in diluted MMH/Ar mixtures and is a significant improvement in predicting the induction delays of concentrated MMH/Ar mixtures.

Progress and Prospective of Nitrogen-Based Alternative Fuels

Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this review, we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the production, distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based molecules in the energy sector, outlining their use as energy carriers in relevant fields.

An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia

A wide-range experimental and theoretical investigation of ammonia gas-phase oxidation is performed, and a predictive, detailed kinetic model is developed.

Ab initio kinetics of the C2H2 + NH2 reaction: a revisited study

This work provides a rigorous detailed kinetic study on the C₂H₂ + NH₂ reaction in a wide range of conditions (T = 250-2000 K & P = 1-76000 Torr). In particular, the composite method W1U was used to construct the potential energy surface on which the kinetic behaviors were characterized within the state-of-the-art master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) framework. Corrections of the hindered internal rotation (HIR) treatment and quantum tunneling effect were included. A clear reaction mechanism shift with respect to both temperature and pressure was revealed via detailed kinetic and species analyses. In particular, bimolecular products (i.e., CH₂[double bond, length as m-dash]C[double bond, length as m-dash]NH + H, CH[triple bond, length as m-dash]CNH₂ + H, CH₃CN + H, CH[triple bond, length as m-dash]C· + NH₃ in the decreasing mole fraction order) can be formed directly from the reactants at high temperature and/or low pressure while they can be produced indirectly via intermediates (e.g., ·CH[double bond, length as m-dash]CHNH₂(cis), ·CH[double bond, length as m-dash]CHNH₂(trans), CH₂[double bond, length as m-dash]C·NH₂,…) at low temperature and/or high pressure. The calculated rate constants are in good agreement with the literature data from ab initio calculations without any adjustment; thus, the proposed temperature- and pressure-dependent rate constants, together with the thermodynamic data of the species involved, can be confidently used for modeling NH₂-related systems under atmospheric and combustion conditions.

Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. I. N2/H2 and NH3/H2 Plasmas

We report a combined experimental/modeling study of microwave activated dilute N2/H2 and NH3/H2 plasmas as a precursor to diagnosis of the CH4/N2/H2 plasmas used for the chemical vapor deposition (CVD) of N-doped diamond. Absolute column densities of H(n = 2) atoms and NH(X(3)Σ(-), v = 0) radicals have been determined by cavity ring down spectroscopy, as a function of height (z) above a molybdenum substrate and of the plasma process conditions, i.e., total gas pressure p, input power P, and the nitrogen/hydrogen atom ratio in the source gas. Optical emission spectroscopy has been used to investigate variations in the relative number densities of H(n = 3) atoms, NH(A(3)Π) radicals, and N2(C(3)Πu) molecules as functions of the same process conditions. These experimental data are complemented by 2-D (r, z) coupled kinetic and transport modeling for the same process conditions, which consider variations in both the overall chemistry and plasma parameters, including the electron (Te) and gas (T) temperatures, the electron density (ne), and the plasma power density (Q). Comparisons between experiment and theory allow refinement of prior understanding of N/H plasma-chemical reactivity, and its variation with process conditions and with location within the CVD reactor, and serve to highlight the essential role of metastable N2(A(3)Σ(+)u) molecules (formed by electron impact excitation) and their hitherto underappreciated reactivity with H atoms, in converting N2 process gas into reactive NHx (x = 0-3) radical species.

On the abundance of non-cometary HCN on Jupiter

Using one-dimensional thermochemical/photochemical kinetics and transport models, we examine the chemistry of nitrogen-bearing species in the Jovian troposphere in an attempt to explain the low observational upper limit for HCN. We track the dominant mechanisms for interconversion of N2-NH3 and HCN-NH3 in the deep, high-temperature troposphere and predict the rate-limiting step for the quenching of HCN at cooler tropospheric altitudes. Consistent with some other investigations that were based solely on time-scale arguments, our models suggest that transport-induced quenching of thermochemically derived HCN leads to very small predicted mole fractions of hydrogen cyanide in Jupiter's upper troposphere. By the same token, photochemical production of HCN is ineffective in Jupiter's troposphere: CH4-NH3 coupling is inhibited by the physical separation of the CH4 photolysis region in the upper stratosphere from the NH3 photolysis and condensation region in the troposphere, and C2H2-NH3 coupling is inhibited by the low tropospheric abundance of C2H2. The upper limits from infrared and submillimetre observations can be used to place constraints on the production of HCN and other species from lightning and thundershock sources.

Growing multiconfigurational potential energy surfaces with applications to X+H2 (X=C,N,O) reactions

A previously developed method, based on a Shepard interpolation procedure to automatically construct a quantum mechanical potential energy surface (PES), is extended to the construction of multiple potential energy surfaces using multiconfigurational wave functions. These calculations are accomplished with the interface of the PES-building program, GROW, and the GAMESS suite of electronic structure programs. The efficient computation of multiconfigurational self-consistent field surfaces is illustrated with the C + H2, N + H2, and O + H2 reactions.