Ammonia Pyrolysis Research Articles

Novel Lewis Acid‐Base Interactions in Polymer‐Derived Sodium‐Doped Amorphous Si−B−N Ceramic: Towards Main‐Group‐Mediated Hydrogen Activation

AbstractInterest is growing in transition metal‐free compounds for small molecule activation and catalysis. We discuss the opportunities arising from synthesizing sodium‐doped amorphous silicon‐boron‐nitride (Na‐doped a‐SiBN). Na+ cations and 3‐fold coordinated BIII moieties were incorporated into an amorphous silicon nitride network via chemical modification of a polysilazane followed by pyrolysis in ammonia (NH3) at 1000 °C. Emphasis is placed on the mechanisms of hydrogen (H2) activation within Na‐doped a‐SiBN structure. This material design approach allows the homogeneous distribution of Na+ and BIII moieties surrounded by SiN4 units contributing to the transformation of the BIII moieties into 4‐fold coordinated geometry upon encountering H2, potentially serving as frustrated Lewis acid (FLA) sites. Exposure to H2 induced formation of frustrated Lewis base (FLB) N−= sites with Na+ as a charge‐compensating cation, resulting in the in situ formation of a frustrated Lewis pair (FLP) motif (≡BFLA⋅⋅⋅Hδ−⋅⋅⋅Hδ+⋅⋅⋅:N−(Na+)=). Reversible H2 adsorption‐desorption behavior with high activation energy for H2 desorption (124 kJ mol−1) suggested the H2 chemisorption on Na‐doped a‐SiBN. These findings highlight a future landscape full of possibilities within our reach, where we anticipate main‐group‐mediated small molecule activation will have an important impact on the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Hydrogen abstraction reactions by NH2 radicals from C3-C6 Cycloalkanes: A theoretical study

Amino (NH2) radicals are crucial in ammonia pyrolysis and oxidation, and their reactions in NH3-dual-fuel combustion are well-recognized. We theoretically investigated the reactions of NH2 radicals with C3-C6 cycloalkanes – a component type of gasoline, using the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. Canonical transition state theory with corrections for hindered internal rotation and Eckart tunneling effects was employed to gain high-pressure limiting rate constants over 500 – 2000 K. Our calculated rate constant for NH2 with cyclohexane matches the experimental measurement, though experimental data for other reactions are limited. The Arrhenius parameters for the studied reactions are also provided.

Ammonia pyrolysis oxidation excited by nanosecond pulsed discharge: Global/fluid models hybrid solution

The kinetic characteristics of plasma-assisted oxidative pyrolysis of ammonia are studied by using the global/fluid models hybrid solution method. Firstly, the stable products of plasma-assisted oxidative pyrolysis of ammonia are measured. The results show that the consumption of NH3/O2 and the production of N2/H2 change linearly with the increase of voltage, which indicates the decoupling of non-equilibrium molecular excitation and oxidative pyrolysis of ammonia at low temperatures. Secondly, the detailed reaction kinetics mechanism of ammonia oxidative pyrolysis stimulated by a nanosecond pulse voltage at low pressure and room temperature is established. Based on the reaction path analysis, the simplified mechanism is obtained. The detailed and simplified mechanism simulation results are compared with experimental data to verify the accuracy of the simplified mechanism. Finally, based on the simplified mechanism, the fluid model of ammonia oxidative pyrolysis stimulated by the nanosecond pulse plasma is established to study the pre-sheath/sheath behavior and the resultant consumption and formation of key species. The results show that the generation, development, and propagation of the pre-sheath have a great influence on the formation and consumption of species. The consumption of NH3 by the cathode pre-sheath is greater than that by the anode pre-sheath, but the opposite is true for OH and O(1S). However, within the sheath, almost all reactions do not occur. Further, by changing the parameters of nanosecond pulse power supply voltage, it is found that the electron number density, electron current density, and applied peak voltages are not the direct reasons for the structural changes of the sheath and pre-sheath. Furthermore, the discharge interval has little effect on the sheath structure and gas mixture breakdown. The research results of this paper not only help to understand the kinetic promotion of non-equilibrium excitation in the process of oxidative pyrolysis but also help to explore the influence of transport and chemical reaction kinetics on the oxidative pyrolysis of ammonia.

Enhanced Room-Temperature NO2 Sensing through Deep Functional Group Hybridization in Nitrogen-Doped Monolayer Ti3C2Tx.

Nitrogen dioxide (NO2) is a significant environmental and human health hazard. Current NO2 sensors often lack sensitivity and selectivity under ambient conditions. This study investigates ammonia pyrolysis modification of monolayer Ti3C2Tx MXene to enhance NO2 detection at room temperature. Nitrogen-doped Ti3C2Tx demonstrates a substantial improvement in sensitivity, with a response of 8.87% to 50 ppm of NO2 compared to 0.65% for the original sensor, representing a 13.8-fold increase. The nitrogen-doped sensor also exhibits superior selectivity and linearity for NO2 under ambient conditions. Theoretical analysis shows that nitrogen incorporation promotes enhanced interaction between Ti3C2Tx and its surface oxygen-containing functional groups through electronic hybridization, resulting in improved adsorption energy (1.80 |eV|) and electron transfer efficiency (0.67 |e|) for NO2, thereby enhancing its gas-sensing performance. This study highlights the potential of ammonia pyrolysis-treated Ti3C2Tx MXene for advancing NO2 sensor technologies with heightened performance at room temperature.

Nitrogen doping in carbon xerogels via ammonia pyrolysis: A case study

Nitrogen-doped carbon xerogels (CXNs) were carbonized across a range of temperatures (400 °C–800 °C) using sol-gel derived resorcinol-formaldehyde xerogel to elucidate their structural evolution during carbonization in ammonia. The investigation extensively probed chemical and structural changes in CXNs at varying carbonization temperatures, employing analytical methods such as X-ray photoelectron spectroscopy, infrared spectroscopy, and energy-dispersive X-ray spectroscopy. These findings were thoroughly compared with the local structure derived from total X-ray scattering and Raman spectroscopy. The results showed a transformation in the local ordering and chemical structure of CXNs, characterized by a reduction in oxidation sites, expansion of C sp2 network structures, leading to the growth of aromatic domains, and the incorporation of nitrogen into the carbon backbone structure. Pyrolysis in ammonia notably influenced the textural structure, resulting in a partial loss of mesopores and an increased micropore volume.

Thermodynamic Evaluation of Hydrogen Production Modes During the Pyrolysis of Ammonia in a Filtration Combustion Moving Bed Reactor

Thermodynamic Evaluation of Hydrogen Production Modes During the Pyrolysis of Ammonia in a Filtration Combustion Moving Bed Reactor

A novel swirl burner with coal co-firing ammonia: Effect of extension length of the dual-channel ammonia pipe on combustion and NOx formation

The coal/ammonia co-firing technology for thermal power generation is a feasible method to effectively reduce CO2 emissions. However, the easy formation of NOx is one of the challenges faced by this technology owing to the high nitrogen content in ammonia. This study proposes a novel swirl burner with a scalable dual-channel ammonia pipe, and evaluates the combustion and NOx formation under different extension lengths of the dual-channel ammonia pipe (i.e., 0 m, 0.4 m, 0.8 m, 1.0 m, 1.2 m). The results show that the extension length of the dual-channel ammonia pipe significantly affects the flow, combustion, and NOx formation. When the extension length of the dual-channel ammonia pipe is above 0.8 m, the ammonia stream can completely penetrate the internal recirculation zone into the high CO environment, which contributes to promoting the ammonia pyrolysis and inhibiting the ammonia oxidation to form NOx. NOx emissions significantly decrease from 846 ppm to 146 ppm as the extension length of the dual-channel ammonia pipe from 0 m to 0.8 m, but change slightly as further increases from 0.8 m to 1.2 m. The carbon content in fly ash decreases from 4.53% to 3.40% as the extension length of the dual-channel ammonia pipe increases from 0 m to 1.0 m. Still, it increases from 3.40% to 3.70% as the extension length of the dual-channel ammonia pipe rises to 1.2 m. Overall, it is reasonable to set the extension length of the dual-channel ammonia pipe at 1.0 m in this studied condition, which can obtain low NOx and low carbon content in fly ash.

Numerical Simulation of Effective Heat Recapture Ammonia Pyrolysis System for Hydrogen Energy

This paper proposes a solution to address the challenges of high storage and transport costs associated with using hydrogen (H2) as an energy source. It suggests utilizing ammonia (NH3) as a hydrogen carrier to produce H2 onsite for hydrogen gas turbines. NH3 offers higher volumetric hydrogen density compared to liquid H2, potentially reducing shipping costs by 40%. The process involves NH3 pyrolysis, which utilizes the heat waste from exhaust gas generated by gas turbines to produce H2 and nitrogen (N2). Numerical simulations were conducted to design and understand the behaviour of the heat recapture NH3 decomposition system. The design considerations included the concept of the number of transfer units and heat exchanger efficiency, achieving a heat recapture system efficiency of up to 91%. The simulation of NH3 decomposition was performed using ANSYS, a commercial simulation software, considering wall surface reactions, turbulent flow, and chemical reaction. Parameters such as activation energy and pre-exponential factor were provided by a study utilizing a nickel wire for NH3 decomposition experiments. The conversion of NH3 reached up to 94% via a nickel-based catalyst within a temperature range of 823 K to 923 K which is the exhaust gas temperature range. Various factors were considered to compare the efficiency of the system, including the mass flow of NH3, operating gauge pressure, mass flow of exhaust gas, among others. Result showed that pressure would not affect the conversion of NH3 at temperatures above 800 K, thus a lower amount of energy is required for a compression purpose in this approach. The conversion is maintained at 94% to 97% when lower activation energy is applied via a ruthenium-based catalyst. Overall, this study showed the feasibility of utilizing convective heat transfer from exhaust gas in hydrogen production by NH3 pyrolysis, and this will further enhance the development of NH3 as the potential H2 carrier for onsite production in hydrogen power generation.

Numerical investigation on pyrolysis and ignition of ammonia/coal blends during co-firing

Co-firing ammonia with coal is a promising and feasible technology for reducing coal-related carbon emissions. Pyrolysis and ignition of ammonia-coal blended fuels are the key steps for flame stability and boiler operation safety throughout the conversion process but remain unclear. In this work, an extended Euler–Lagrange framework coupled to detailed solid-phase pyrolysis kinetics and gas-phase reactions mechanism is introduced and validated against experimental results for ammonia-coal co-firing in a two-stage flat flame burner. First, the CRECK-S coal pyrolysis model was validated with TGA experiments and the CPD model at different heating rates to determine parameters for a competing two-step model for CFD simulations. Second, a detailed gas-phase mechanism with 116 species and 1513 elementary reactions was derived from the volatile and ammonia reaction mechanisms. Thirdly, the co-firing simulations were validated for the ignition delay time for various ammonia co-firing ratios. The results show that increasing the co-firing ratios from 0.0 to 1.0 results in gradually increasing ignition delay times in a low-oxygen atmosphere. Further analysis demonstrates that adding ammonia accelerates the coal particle heating rate due to the reduced coal particle number density and ammonia reaction induced gas-phase temperature increase, and thus the coal devolatilization rate is increased. The latter plays a dominant role. Hydrogen produced from ammonia pyrolysis is negligible, so ammonia predominantly participates in the ignition. Time scale analysis shows that homogeneous ignition is dominant during the ignition process. The presence of ammonia inhibits the inward oxygen diffusion and causes the reaction zone to move away from the pulverized coal particle flow. The oxygen diffusion inhibition and low reactivity of ammonia compared to volatiles lead to an increase in ignition delay time for ammonia co-firing, even though pulverized coal devolatilization is accelerated.

Preparation of nitrogen-containing compounds and nitrogen-doped biochar via nitrogen-rich pyrolysis coupled with ammonia source impregnation

Using lignocellulose to produce nitrogen-rich bio-oil and nitrogen-doped biochar in the presence of nitrogen-carrying bodies has become a research hotspot in recent years for high-value conversion and utilization of biomass. Different nitrogen carriers significantly influence biomass pyrolysis and conversion process and products. In this study, a new strategy for the preparation of both nitrogen-rich bio-oil (high-value N-containing compounds (NCCs)) and N-doped biochar by ammonia impregnation treatment coupled with Zeolite Socony Mobil Five (HZSM-5) via N-enriched pyrolysis was proposed. The influence of amine type (formamide, urea, melamine and dicyandiamide) and concentration on the product content, distribution, properties and evolution mechanism was also explored. The results showed that the amine treatment significantly reduced the activation energy (from 23.71 kJ/mol to 12.35–20.82 kJ/mol) and N content (from 7.42% to 6.05–7.29%) due to destruction of the crystalline phase and amine volatilization but increased the amount of char formed (from 18.15% to 21.47–24.06%). The amino groups derived from four different ammonia sources (formamide, urea, melamine and dicyandiamide) reacted with biomass-derived O-containing compounds to form pyridinic- and graphitic-like biochar due to NH3 etching and activation, and the enhanced adsorption capacities decreased in the order melamine (107.95 m2/g) > urea (96.84 m2/g) > dicyandiamide (79.46 m2/g) > formamide (74.12 m2/g) > biochar (35.63 m2/g)) due to the higher specific surface areas, pore volumes, graphitization degrees and enrichment of the O/N-containing functional groups. Additionally, higher concentrations of formamide coupled with the HZSM-5 catalyst noticeably boosted the NCCs content and pyrrole selectivity. The maximum NCCs content and pyrrole selectivity were 82.97% and 52.45%, respectively, and these were obtained via catalytic pyrolysis with HZSM-5 when the formamide concentration was 15%. Furthermore, formamide preferentially generated NCCSs and pyrroles via the Maillard reaction, and urea facilitated amine formation. This research provides useful background knowledge and a novel process for transformation and recovering high value-added NCCs products by N-rich pyrolysis of ammonia impregnated biomass.

Hydrogen-bond induced and hetero coupling dual effects in N-doped carbon coated CrN/Ni nanosheets for efficient alkaline freshwater/seawater hydrogen evolution

Developing efficient and robust non-precious-metal-based hydrogen evolution reaction (HER) catalysts is highly desirable but remains quite challenging for alkaline freshwater/seawater electrolysis. In the present study, we report a theory-guided design and synthesis of a nickel foam (NF) supported N-doped carbon-coated (NC) nickel (Ni)/chromium nitride (CrN) nanosheets (NC@CrN/Ni) as a highly active and durable electrocatalyst. Our theoretical calculation firstly reveals that CrN/Ni heterostructure can greatly promote the H2O dissociation via hydrogen-bond induced effect, and the N site can be optimized by hetero coupling to achieve a facile hydrogen associative desorption, thereby significantly boosting alkaline HER. Guided by theoretical calculation, we prepared the nickel-based metal–organic framework as a precursor, and introduced the Cr by the subsequent hydrothermal treatment, finally obtained the target catalyst by ammonia pyrolysis. Such a simple process ensures the exposure of abundant accessible active sites. Consequently, the as-prepared NC@CrN/Ni catalyst exhibits outstanding performance in both alkaline freshwater and seawater, with the respective overpotential of only 24 and 28 mV at a current density of 10 mA cm−2, respectively. More impressively, the catalyst also possesses superior durability in the constant-current test of 50 h at the different current densities of 10, 100, and 1000 mA cm−2.

Co4N embedded nitrogen doped carbon with 2D/3D hybrid structure as sulfur host for room-temperature sodium-sulfur batteries

Room temperature sodium-sulfur (RT Na-S) batteries demonstrate a high theoretical energy density, but their practical application is limited by the sodium polysulfides (NaPSs) shuttling and slow redox kinetics. Herein, we report the design of the dimensional hybrid structure of 2D ZIF-67 nanosheets and 3D ZIF-67 polyhedron on the surface of carbon fiber by a pseudomorphic replication approach. After pyrolysis in ammonia, a hybrid structure composed of 2D N-doped carbon nanosheets and 3D N-doped carbon nano-polyhedrons embedded with Co4N nanoparticles (denoted as 2D/3D Co4N-NC@CC) is designed as a sulfur host for RT Na-S battery. The complex structure of 2D/3D composite guarantees abundant adsorption and catalytic sites, accelerating electrons/mass transport abilities compared with single 2D or 3D structure, in which Co4N can effectively catalyze the conversion of NaPSs and inhibit “shuttling effect”. Consequently, the RT Na-S cell with 2D/3D Co4N-NC@CC cathode exhibits extra-high discharge capacity of 466 mAh g−1 at 3.0 C and an outstanding long-term cycling stability (592 mAh g−1 at 1.0 C after 1000 cycles). The hybrid 2D/3D Co4N-NC@CC with excellent electrochemical performance represents a potential candidate for developing advanced RT Na-S batteries and other energy storage systems.

Numerical Investigation of Exergy Loss of Ammonia Addition in Hydrocarbon Diffusion Flames.

In this paper, a theoretical numerical analysis of the thermodynamics second law in ammonia/ethylene counter-flow diffusion flames is carried out. The combustion process, which includes heat and mass transfer, as well as a chemical reaction, is simulated based on a detailed chemical reaction model. Entropy generation and exergy loss due to various reasons in ammonia/ethylene and argon/ethylene flames are calculated. The effects of ammonia addition on the thermodynamics efficiency of combustion are investigated. Based on thermodynamics analysis, a parameter, the lowest emission of pollutant (LEP), is proposed to establish a relationship between the available work and pollutant emissions produced during the combustion process. Chemical reaction paths are also analyzed by combining the chemical entropy generation, and some important chemical reactions and substances are identified. The numerical results reveal that ammonia addition has a significant enhancement on heat transfer and chemical reaction in the flames, and the total exergy loss rate increases slightly at first and then decreases with an increase in ammonia concentration. Considering the factors of thermodynamic efficiency, the emissions of CO2 and NOx reach a maximum when ammonia concentration is near 10% and 30%, respectively. In terms of the chemical reaction path analysis, ammonia pyrolysis and nitrogen production increase significantly, while ethylene pyrolysis and carbon monoxide production decrease when ammonia is added to hydrocarbon diffusion flames.

NaClO-induced sodium-doped cyano-rich graphitic carbon nitride nanosheets with nitrogen vacancies to boost photocatalytic hydrogen peroxide production

Graphitic carbon nitride (g-C3N4) has shown promising potential application in photocatalytic production of hydrogen peroxide (H2O2) due to its suitable electronic band structure and facile preparation. Nevertheless, the ability of H2O2 production is limited by the unsatisfactory light utilization and the low specific surface area. In the present work, we developed a NaClO-assisted ammonia pyrolysis method to achieve the simultaneous introduction of sodium doping and cyano groups with nitrogen vacancies into the framework of g-C3N4 by the synergistic effect of Na+ intercalation and ClO− oxidation. Moreover, the easy removal of NaClO-derived NaCl can further increase the specific surface area and exposed active sites after water washing. All the features above are beneficial to extend the light absorption and improve the separation efficiency and redox ability of photo-generated electrons and holes over the optimal catalyst, which contribute to the high H2O2 yield of 415.5 μM within 4 h and excellent cyclic stability.

Challenges in Kinetic modeling of ammonia pyrolysis

Ammonia pyrolysis reactions have implications for its ignition and oxidation in engines and gas turbines. In the present work, the chemistry of ammonia pyrolysis is investigated by kinetic modeling and theory. Rate constants for key reactions are carefully evaluated based on available experimental and theoretical results. The high pressure limit k1,∞ for NH2 + H (+M) ⇄ NH3 (+M) (R1) is calculated to be essentially the collision frequency, indicating that dissociation of ammonia in combustion processes will be at or close to the low pressure limit even at engine and gas turbine conditions. The chemical kinetic model is validated against reported shock tube measurements of NH3, NH2, and NH in ammonia pyrolysis. Predictions are in good agreement with observations for dilute conditions (≤ 0.5% NH3), but the model appears to underpredict the NH3 consumption rate at longer times in less dilute mixtures. At short reaction times, thermal dissociation of NH3, together with the NH3 + H reaction, controls conversion. At longer times, secondary reactions involving NH2 and NH become important due to their impact on the radical pool. Predictions become sensitive to formation and consumption of diazene (tHNNH and cHNNH). Several of the key steps in the ammonia pyrolysis mechanism are radical-radical reactions that are difficult to measure accurately and challenging to calculate theoretically, and a more comprehensive experimental characterization is desirable to support further model development.

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.

Modeling ammonia-fueled co-flow dual-channel protonic-ceramic fuel cells

ABSTRACT This paper reports the model development for a dual-channel protonic-ceramic fuel cell (PCFC) operating on ammonia fuel. The model considers the coupled interactions of several physical and chemical processes, including three-dimensional heat conduction within the bipolar plates and the membrane-electrode assembly (MEA), one-dimensional flow within the fuel and air channels, detailed heterogeneous catalytic reactions within the porous composite anode structure, Butler–Volmer representation of the charge-transfer chemistry, and Nernst–Planck transport of three charged defects (protons, oxygen vacancies, and small polarons) within the dense electrolyte membrane. The membrane-electrode assembly is composed of a Ni-BCZYYb () anode, a BCZYYb electrolyte membrane, and a BCFZY () cathode. Chemical and physical parameters for the MEA model are established using previously published button-cell data. One aspect of the study is to investigate the partial ammonia decomposition upstream of the fuel cell. Such fuel cracking increases the H content of the fuel entering the PCFC, which may have benefits. However, endothermic ammonia pyrolysis within the composite anode structure assists with thermal control of the cell. The dual-channel model can be considered as the unit cell of a full fuel-cell stack.

An experimental and modeling study of ammonia pyrolysis

Ammonia (NH3) is a promising carbon-free fuel and a hydrogen carrier. In recent years, there has been a large number of experimental and numerical studies to understand the chemical kinetics of NH3 in moderate to extremely complex systems. This study focused on understanding the chemical kinetics of NH3 in a simple system (pyrolysis). Shock-tube experiments were performed to monitor the NH3 time history profiles during pyrolysis, with and without the presence of H2, using a new laser absorption diagnostic near 10.4 µm. The pyrolysis experiments were conducted for mixtures of ∼ 0.5% NH3/Ar and ∼ 0.42% NH3/2% H2/Ar behind reflected shock waves, near atmospheric pressure, and over a temperature range of 2100–3000 K. Using the data from the present study as a guide, along with NH3 pyrolysis data from the literature, a detailed chemical kinetics mechanism for NH3 pyrolysis is proposed herein. This mechanism was assembled using available reaction rate constants from the literature. The mechanism showed excellent agreement with the experimental results, as well as with the literature data. Additionally, an assessment of 15 detailed NH3 chemical kinetics mechanisms on their capabilities of predicting the new pyrolysis experiments was performed. The assessment showed that these literature mechanisms yield significantly different predictions, with only one model producing acceptable results for the majority of the NH3 pyrolysis experiments. The effect of pyrolysis reactions on the prediction of oxidation data was investigated by updating the pyrolysis sub-mechanism of selected literature models with the reactions of the present pyrolysis model. The prediction of the updated literature models significantly improved for ignition delay time and flame speed literature data, indicating the importance of pyrolysis reactions for high-temperature oxidation chemistry.

An evolutionary, data-driven approach for mechanism optimization: theory and application to ammonia combustion

In this work, we propose a novel data-driven approach for detailed kinetic mechanisms optimization. The approach is founded on a curve matching-based objective function and includes a methodology for the optimisation of pressure-dependent reactions via logarithmic interpolation (PLOG format). In order to highlight the advantages of the new formulation of the objective function, a comparison with L1 and L2 norm is performed. The selection of impactful reactions is carried out by introducing a Cumulative Impact Function (CIF), while an Evolutionary Algorithm (EA) is adopted for the optimization. The capabilities of the proposed methodology were demonstrated using a database of ~635 experimental datapoints on ammonia combustion, covering standard targets like ignition delay times, speciation and laminar flame speed. The optimization was carried out starting from a recently published mechanism, describing ammonia pyrolysis and oxidation, largely developed using first-principles calculation of rate constants. After the selection of the 24 most impactful reactions, the related 101 normalized Arrhenius parameters were simultaneously varied, within their uncertainty bounds. Their uncertainty bounds were taken from the literature, when available, or estimated according to the level of theory adopted for the determination of the rate constant. Hence, we also provide guidelines to estimate uncertainty for reaction rate constants derived from first principles calculations using well consolidated computational protocols as a reference. The optimized mechanism was found to improve the nominal one, showing a satisfactory agreement over the entire range of operating conditions. Moreover, the use of ‘curve matching’ indices was found to outperform the adoption of L1 and L2 norms. The comparison between the nominal mechanism and the one optimized via curve matching allowed a clear identification of different critical reaction pathways for different experimental targets. From this perspective, the methodology proposed herein can find further application as a useful design-of-experiments tool for an accurate evaluation of crucial kinetic constants, thus driving further mechanism improvement.

Oxidation and pyrolysis of ammonia mixtures in model reactors

The present work was devoted to an experimental characterization of NH3 oxidation and pyrolysis processes in model reactors. The oxidation tests were performed in a Jet Stirred Flow Reactor (JSFR) for stoichiometric ammonia/oxygen/argon and ammonia/oxygen/argon/water mixtures, as a function of mixture inlet temperature (Tin), at fixed pressure and residence time. Following, the behavior of ammonia/oxygen/argon mixture was analyzed in two tubular laminar flow reactors (LFRs) made of different materials (quartz and alumina), to evaluate potential surface heterogeneous effects.Pyrolysis tests were performed in both the reference model reactors, by substituting oxygen with argon.Experimental results realized in the JSFR allowed to identify three different kinetics regimes in ammonia oxidation: low (Tin < 1130 K), intermediate (1130 < Tin < 1250 K) and high temperatures (Tin > 1250 K). Surface heterogeneous reactions seemed to be negligible on ammonia reactivity and hydrogen profiles, but some effect could be observed on NOx concentration in the low-intermediate temperature range. Experiments realized in presence of water suggested that such species may passivate the surface, minimizing heterogeneous effects.Experimental tests realized in the LFRs confirmed these results, as hydrogen and oxygen profiles were independent of the reactor material, while NOx concentrations exhibited significant changes. Experimental results suggested that heterogeneous reactions were more relevant under pyrolytic conditions.Numerical simulations were performed with different kinetic mechanisms available in literature to value their capability to describe ammonia chemistry. None of them could accurately reproduce the experimental data for ammonia oxidation process, while they completely failed to predict ammonia pyrolysis features.