An experimental and modeling study on auto-ignition of ammonia in an RCM with N2O and H2 addition

Deep insights into the combustion kinetics of ammonia (NH₃) can facilitate its application as a promising carbon-free fuel. Due to the low reactivity of NH₃, experimental data of NH₃ combustion can only be obtained within a limited range. In this work, nitrous oxide (N₂O) and hydrogen (H₂) were used as additives to investigate NH₃ auto-ignition in a rapid compression machine (RCM). Ignition delay times for NH₃, NH₃/N₂O blends, and NH₃/H₂ blends were measured at 30 bar, temperatures from 950 to 1437 K. The addition of N₂O and H₂ ranged from 0 to 50% and 0 to 25% of NH₃ mole fraction, respectively. Time-resolved species profiles were recorded during the auto-ignition process using a fast sampling system combined with a gas chromatograph (GC). An NH₃ combustion model was developed, in which the rate constants of key reactions were constrained by current experimental data. The addition of N₂O affected the ignition of NH₃ primarily through the decomposition of N₂O (N₂O (+M) = N₂ + O (+M), R1) and direct reaction between N₂O and NH₂ (N₂H₂ + NO = NH₂ + N₂O, R2). The rate constant of R2 was constrained effectively by experimental data of NH₃/N₂O mixtures. Two-stage ignition behaviors were observed for NH₃/H₂ mixtures, and the corresponding first-stage ignition delay times were reported for the first time. Experimental species profiles suggested the first-stage ignition resulted from the consumption of H₂. The oxidation of H₂ provided extra HO₂ radicals, which promoted the production of OH radicals and initiated first-stage ignition. Reactions between HO₂ radicals and NH₃/NH₂ dominated the first-ignition delay times of NH₃/H₂ mixtures. Moreover, the first-stage ignition led to the fast production of NO₂, which acted as a key intermediate and affected the following total ignition. Consequently, the reaction NH₂ + NO₂ = H₂NO + NO (R3) was constrained by total ignition delay times.     

Crystal structure of di-(L-serine) phosphate monohydrate [C3O3NH7]2H3PO4H2O

The crystal structure of di-(L-serine) phosphate monohydrate [C₃O₃NH₇]₂H₃PO₄H₂O is determined by single-crystal x-ray diffraction. The intensities of x-ray reflections are measured at temperatures of 295 and 203 K. The crystal structure is refined using two sets of intensities. It is established that, in the structure, symmetrically nonequivalent molecules of L-serine occur in two forms, namely, the monoprotonated positively charged molecule CH₂(OH)CH(NH₃)⁺COOH and the zwitterion CH₂(OH)CH(NH₃)⁺COO^?, which are linked with each other and with the H₂PO ^?₄ ion through a hydrogen-bond system involving water molecules.

     

Experimental and modeling study of water time histories during H2S-N2O combustion in a shock tube

With the interest in directly burning sour gas in gas turbines, and the fact that even small amounts of H₂S or its combustion products can alter combustion characteristics, many research studies have been performed to better understand the combustion chemistry of H₂S. In the present study, the water formation was followed by laser absorption with N₂O as an oxidant, instead of O₂. Nitrous Oxide being essentially consumed via N₂O (+M) ⇌ N₂ + O (+M), the water formation via the H₂S + O route can then be probed to further validate the models. Three H₂S/N₂O mixtures diluted in 98% Ar were studied to cover the following range of equivalence ratios: 0.5, 1.0, and 2.0, over a wide range of temperatures (1580–1940 K) around atmospheric pressure. A chemical kinetic model was then developed first by validating the base N₂O kinetics mechanism, then by investigating the experimental results presented herein. The N₂O kinetics mechanism was updated with recent work on the low- and high-pressure limits for N₂O decomposition (N₂O (+M) ⇌ N₂ + O (+M)) as well as a review of rate coefficients for N₂O + H ⇌ N₂ + OH from the literature. Good agreement is shown for H₂/N₂O mixtures. Updates were then made to the H₂S kinetics mechanism, specifically, an update from the literature on SO₂ + H (+M) ⇌ SO + OH (+M) and an adjustment to SO + SH ⇌ S₂ + OH. Additionally, reactions between SH and N₂O were determined using W1BD data and transition state theory which required the addition of an NNS sub-mechanism. With these updates, the mechanism provides good agreement with the H₂S/N₂O experiments.     

Diode-laser absorption measurements of CO2, H2O, N2O, and NH3 near 2.0 μm

2 , H₂O, N₂O, and NH₃ concentrations in various flowfields using absorption spectroscopy and extractive sampling techniques. An external-cavity diode laser with a tuning range of 1.953–2.057 μm was used to record absorption lineshapes from measured transitions in the CO₂ 4ν₂ ⁰+ν₃, ν₁+2ν₂ ⁰+ν₃, and 2ν₁+ν₃ bands, H₂O ν₂+ν₃and ν₁+ν₂ bands, N₂O 2ν₁+4ν₂ ⁰, ν₂ ¹+2ν₃, 3ν₁+2ν₂ ⁰, and 4ν₁ bands, and NH₃ν₁+ν₄ and ν₃+ν₄ bands. Measured CO₂, H₂O, and N₂O survey spectra were compared to calculations to verify the HITRAN96 database and used to determine optimum transitions for species detection. Individual lineshape measurements were used to determine fundamental spectroscopic parameters including the line strength, line-center frequency, and self-broadening coefficient of the probed transition. The results represent the first measurements of CO₂, H₂O, N₂O, and NH₃ absorption near 2.0 μm using room-temperature near-IR diode lasers. Received: 12 March 1998/Revised version: 7 May 1998     

Shock tube study of the interaction between ammonia and nitric oxide at high temperatures using laser absorption spectroscopy

The interaction between ammonia (NH₃) and nitric oxide (NO) at high temperatures is studied in this work using a shock tube combined with laser absorption diagnostics. The system simultaneously measured the NH₃ and NO time-histories during the reaction processes of the shock-heated NH₃/NO/CO/Ar mixtures (NH₃:NO ≈ 0.9:1.0 and 1.4:1.0). The absorption cross-sections of NH₃ near 1122.10 cm^–1 and NO at 1900.52 cm^–1 (characterized in this study) were used for measuring NH₃ and NO time-histories with the temperature measured by two CO absorption lines. The measured NH₃ and NO time-histories at 1614–1968 K and 2.4–2.8 atm were compared with predictions of seven recent kinetics models. The predictions that based on different mechanisms are very different and the measured profiles are within the range of the predictions. The Glarborg, NUI Galway Syngas-NOx, and Mathieu mechanisms give the closest predictions to the measurements. Kinetics analyses indicate that the NH₃ and NO consumption rates are extremely sensitive to the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH, which are more reliably represented in the Glarborg and NUI Galway Syngas-NOx mechanisms. The performances of Glarborg mechanisms at lower initial temperatures can be apparently improved by revising the rate constants and branching ratio of NH₂ + NO = N₂ + H₂O and NH₂ + NO = NNH + OH. These two reactions are also the primary pathways for NO reduction and NH₃ is mainly consumed via NH₃ + OH = NH₂ + H₂O and NH₃ + H = NH₂ + H₂. Trace amounts of NO₂ and N₂O impurities decompose to form O radical followed by the generation of OH radical via H-abstraction reactions, which significantly affects the predictions of NH₃ and NO according to kinetics analyses.     

Chalcogen based treatment of InAs with [(NH4)2S/(NH4)2SO4]

A sulphur based chemical, ([(NH₄)₂S/(NH₄)₂SO₄]) to which S has been added not previously reported for the treatment of (111)A InAs surfaces is introduced and benchmarked against the commonly used passivants Na₂S·9H₂O and ((NH₄)₂S + S), using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). It has been found that the native oxide layer present on the InAs surface is more effectively removed when treated with ([(NH₄)₂S/(NH₄)₂SO₄] + S) than with ((NH₄)₂S + S) or Na₂S·9H₂O. AES depth profiles of the sulphurized layers revealed the formation of a thin (less than 8.5 nm) In–S surface layer for both ((NH₄)₂SO₄ + S) and ([(NH₄)₂S/(NH₄)₂SO₄] + S) treatments. No evidence for the formation of As―S bonds was found. Treatment with ([(NH₄)₂S/(NH₄)₂SO₄] + S) also affected a significant improvement compared to the more established sulphur treatments in the surface morphology of the otherwise poor as-received n-InAs (111)A surface.     

Measurements of H2O2 in low temperature dimethyl ether oxidation

H₂O₂ is one of the most important species in dimethyl ether (DME) oxidation, acting not only as a marker for low temperature kinetic activity but also responsible for the “hot ignition” transition. This study reports, for the first time, direct measurements of H₂O₂ and CH₃OCHO, among other intermediate species concentrations in helium-diluted DME oxidation in an atmospheric pressure flow reactor from 490 to 750 K, using molecular beam electron-ionization mass spectrometry (MBMS). H₂O₂ measurements were directly calibrated, while a number of other species were quantified by both MBMS and micro gas chromatography to achieve cross-validation of the measurements. Experimental results were compared to two different DME kinetic models with an updated rate coefficient for the H + DME reaction, under both zero-dimensional and two-dimensional physical model assumptions. The results confirm that low and intermediate temperature DME oxidation produces significant amounts of H₂O₂. Peroxide, as well as O₂, DME, CO_, and CH₃OCHO profiles are reasonably well predicted, though profile predictions for H₂/CO₂ and CH₂O are poor above and below ～625 K, respectively. The effect of the collisional efficiencies for the H + O₂ + M = HO₂ + M reaction on DME oxidation was investigated by replacing 20% He with 20% CO₂. Observed changes in measured H₂O₂ concentrations agree well with model predictions. The new experimental characterizations of important intermediate species including H₂O₂, CH₂O and CH₃OCHO, and a path flux analysis of the oxidation pathways of DME support that kinetic parameters for decomposition reactions of HOCH₂OCO and HCOOH directly to CO₂ may be responsible for model under-prediction of CO₂. The H abstraction reactions for DME and/or CH₂O and the unimolecular decomposition of HOCH₂O merit further scrutiny towards improving the prediction of H₂ formation.     

Synthesis and characterization of V3O7⋅H2O nanobelts

V ₃O₇?H₂O nanobelts were prepared by a hydrothermal method at 190 ^°C using V ₂O₅?nH₂O gel and H₂C₂O₄?2H₂O as starting agents. The nanobelts obtained have diameters ranging from 40 to 70 nm with lengths of up to several micrometers. Their morphology and structure have been characterized by XRD, SEM, TEM and IR spectroscopy. The effect of the annealing temperature on the morphology of the resulting product has been investigated. XRD and SEM showed that thermal annealing of the V ₃O₇?H₂O sample in air led to the collapse of the V ₃O₇?H₂O nanobelt structure and the convert into V ₂O₅ nanobelts. For the first time V ₂O₅ nanobelts with an ultrahigh aspect-ratio have been obtained in air. Furthermore, electrical conductivity and static magnetic susceptibility measurements have been carried out.     

Adsorption and diffusion behaviors of H2, H2S,NH3, CO and H2O gases molecules on MoO3 monolayer: A DFT study

Molybdenum trioxide (MoO₃) with α-phase is a promising material for gas sensing because of its high sensitivity, fast response and thermodynamic stability. To probe the mechanism of superior gas detection ability of MoO₃ monolayer, the adsorption and diffusion of H₂, H₂S, NH₃, CO and H₂O molecules on two-dimensional (2D) MoO₃ layer are studied via density functional theory (DFT) calculations. Based on calculated adsorption energies, density of states, charge transfer, diffusion barriers and diffusion coefficient, MoO₃ shows a superior sensitive and fast response to H₂ and H₂S than CO, NH₃, H₂O, which is consistent with experimental conclusions. Moreover, the response of MoO₃ to H₂S and H₂ will be obviously enhanced at high gas concentration, and the incorporation of H₂ and H₂S results in an obvious increasing in DOS near Fermi level. Our analysis provides a conceptual foundation for future design of MoO₃-based gas sensing materials.     

The HO4H → O3 + H2O reaction catalysed by acidic,neutral and basic catalysts in the troposphere

A detailed effects of catalyst X (X?=?H₂O, (H₂O)₂, NH₃, NH₃···H₂O, H₂O···NH₃, HCOOH and H₂SO₄) on the HO₄H → O₃?+?H₂O reaction have been investigated by using quantum chemical calculations and canonical vibrational transition state theory with small curvature tunnelling. The calculated results show that (H₂O)₂-catalysed reactions much faster than H₂O-catalysed one because of the former bimolecular rate constant larger by 2.6–25.9 times than that of the latter one. In addition, the basic H₂O···NH₃ catalyst was found to be a better than the neutral catalyst of (H₂O)₂. However it is marginally less efficient than the acidic catalysts of HCOOH, and H₂SO₄. The effective rate constant (k'_t) in the presence of catalyst X have been assessed. It was found from k'_t that H₂O (at 100% RH) completely dominates over all other catalysts within the temperature range of 280–320?K at 0?km altitude. However, compared with the rate constant of HO₄H → H₂O?+?O₃ reaction, the k _eff values for H₂O catalysed reaction are smaller by 1–2 orders of magnitude, indicating that the catalytic effect of H₂O makes a negligible contribution to the gas phase reaction of HO₄H → O₃?+?H₂O.

Highlights

• A detailed effects of catalyst of H₂O, (H₂O)₂, NH₃, NH₃···H₂O, H₂O···NH₃, HCOOH and H₂SO₄ on the HO₄H → O₃?+?H₂O reaction has been performed.

• From energetic viewpoint, H₂SO₄ exerts the strongest catalytic role in HO₄H → O₃?+?H₂O reaction as compared with the other catalysts.

• At 0 km altitude H₂O (at 100% RH) completely dominates over all other catalysts within the temperature range of 280–320 K.

• HO₄H → H₂O?+?O₃ reaction with H₂O cannot be compete with the reaction without catalyst, due to the fact that the effective rate constants in the presence of H₂O are smaller.

     

NH3 decomposition in a single-chamber proton conducting cell

The catalytic and electrocatalytic decomposition of NH₃ was studied at 350–650 °C and atmospheric total pressure, using a single-chamber H⁺ conducting cell-reactor. The H⁺ conductor was a strontia–ceria–ytterbia (SCY) perovskite of the form: SrCe_0.95Yb_0.05O_3−a. A Ru film was used as catalyst-electrode. The effects of imposed currents, temperature and inlet gas composition on the reaction rate were examined.     

A photoelectron and energy-loss spectroscopy study of Ti and its interaction with H2, O2, N2 and NH3

A unified electron spectroscopic study of polycrystalline Ti and its interaction with H₂, O₂, N₂, and NH₃ are described. Auger electron spectroscopy (AES), electron energy-loss spectroscopy (ELS), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) are combined to provide detailed information about the electronic structure of the titanium surface and its interaction with these adsorbates. X-ray and ultraviolet photoelectron spectra and electron energy-loss spectra are presented for the clean titanium surface, and following exposure to H₂, O₂, N₂ and NH₃. Spectral assignments are provided in each case. The electron spectra of oxygen exposed Ti and nitrogen sputtered Ti are quite similar, and are interpreted with reference to band structure calculations for TiO and TiN. Electron spectroscopy indicates essentially complete dissociative adsorption of NH₃ on the clean titanium surface.     

Shock tube/laser absorption measurement of the rate constant of the reaction: H2O2 + CO2 2OH + CO2

We address the role of the linear mixing rule in the kinetics of the H₂O₂ decomposition system by reporting the rate constant for H₂O₂ + M = 2OH + M (M = Ar and CO₂) in the temperature range of 1087–1234 K at low pressures in a mixture of 20% CO₂ in Argon. The reaction rate constant was inferred from H₂O concentrations monitored by using a laser-absorption spectroscopy-based water diagnostic. To the best of our knowledge, this is the first measurement of the rate constant of this reaction in a mixture to be reported in literature. A significant discrepancy was found between the rate constants derived using the traditional linear mixing rule and the reduced pressure linear mixing rule. This discrepancy can have serious implications on the predictive accuracy of these kinetic models, especially under conditions relevant to the operation of supercritical CO₂ (sCO₂) power cycles that rely on oxy-fuel combustion in a working fluid comprised almost entirely of CO₂.     

A high efficiency NH3/H2O absorption power cycle

A work producing cycle has been developed showing a thermodynamic efficiency considerably higher than that of the Rankine cycle. The new cycle employs a mixture of H₂O and NH₃ as the working fluid and uses an absorption process similar to that of absorption refrigerators. Its advantage over existing power cycles working with the same mixture (i.e. the Kalina cycle) is simplicity as far as devices, construction, operation and maintenance are concerned. For the detailed calculation of the proposed cycle a method has been developed, which employs analytical functions describing the thermodynamic properties of the NH₃/H₂O mixture. The proposed cycle has been compared with Rankine cycles working at the same temperature levels. For fixed upper (i.e. superheating) and lower (i.e. condensation) temperatures, the new cycle shows an efficiency 20% higher than that of the Rankine cycle if the boiling temperature is high, while for low boiling temperatures the superiority of the proposed cycle is much more pronounced. A parametric study has also been conducted for the new cycle, wwhich showed, inter alia, that the optimum difference between the mass fractions of the rich and weak solution is about 0.1 kg NH₃/kg mixture.     

Dispersion behavior and thermal conductivity characteristics of Al2O3–H2O nanofluids

Nanofluid is a kind of new engineering material consisting of solid nanoparticles with sizes typically of 1–100 nm suspended in base fluids. In this study, Al₂O₃–H₂O nanofluids were synthesized, their dispersion behaviors and thermal conductivity in water were investigated under different pH values and different sodium dodecylbenzenesulfonate (SDBS) concentration. The sedimentation kinetics was determined by examining the absorbency of particle in solution. The zeta potential and particle size of the particles were measured and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to calculate attractive and repulsive potentials. The thermal conductivity was measured by a hot disk thermal constants analyser. The results showed that the stability and thermal conductivity enhancements of Al₂O₃–H₂O nanofluids are highly dependent on pH values and different SDBS dispersant concentration of nano-suspensions, with an optimal pH value and SDBS concentration for the best dispersion behavior and the highest thermal conductivity. The absolute value of zeta potential and the absorbency of nano-Al₂O₃ suspensions with SDBS dispersant are higher at pH 8.0. The calculated DLVO interparticle interaction potentials verified the experimental results of the pH effect on the stability behavior. The Al₂O₃–H₂O nanofluids with an ounce of Al₂O₃ have noticeably higher thermal conductivity than the base fluid without nanoparticles, for Al₂O₃ nanoparticles at a weight fraction of 0.0015 (0.15 wt%), thermal conductivity was enhanced by up to 10.1%.     

The formation and stability of co-crystalline NH3 · C2H2 aerosol particles

NH₃/C₂H₂ aerosols were formed in a bath gas cooling cell at cryogenic temperatures and studied using infrared spectroscopy. The infrared band shapes associated with these particles were quite different from those of the pure solids and vibrational shifts of up to 100 wavenumbers were observed. The phase of these particles was identified as a NH₃?·?C₂H₂ co-crystal and was found to be thermodynamically stable with respect to the solids of pure NH₃ and C₂H₂. Experiments with core–shell structures demonstrated that the co-crystalline phase could also be formed through a solid state reaction of pure NH₃ and C₂H₂ in aerosol particles. The results of this work are relevant to planetary atmospheres where both NH₃ and C₂H₂ are present, as the co-crystalline phase can readily form in mixed particles of these substances and its optical properties cannot be understood by simply considering the pure substances.     

NH3NO interaction at low-temperatures: An experimental and modeling study

The present work provides new insight into NH₃NO interaction under low-temperature conditions. The oxidation process of neat NH₃ and NH₃ doped with NO (450, 800 ppm) was experimentally investigated in a Jet Stirred Flow Reactor at atmospheric pressure for the temperature range 900–1350 K. Results showed NO concentration is entirely controlled by DeNO_x reactions in the temperature range 1100–1250 K, while NH₃NO interaction does not develop through a sensitizing NO effect, for these operating conditions.A detailed kinetic model was developed by systematically updating rate constants of controlling reactions and declaring new reactions for N₂H₂ isomers (cis and trans). The proposed mechanism well captures target species as NO and H₂ profiles. For NH₃NO mixtures, NO profiles were properly reproduced through updated DeNO_x chemistry, while NH₂ recombination reactions were found to be essential for predicting the formation of H₂. The role of ammonia as a third-body species is implemented in the updated mechanism, with remarkable effects on species predictions. For neat NH₃ mixture, the reaction H+O₂(+M)=HO₂(+M) was crucial to predict NO formation via the reaction NH₂+HO₂H₂NO+OH.     

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH₃ in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH₃ dissociation was varied from 0 to 25% (0 to 20%v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH₃ in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH₃ dissociation. At 25% NH₃ dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH₃ in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH₃. The production of key radicals including OH, H, O, and NH₂ was enhanced in the presence of H₂ and thus the conversion of NH to NO and then NO to N₂ near LFL and the conversion of NH₂ and NO to N₂ near UFL leading to wider flammability limits.     

Collision-induced dissociation of H 3 + ions

For collision energies between 100 and 500 eV the collision induced dissociation of H ₃ ⁺ colliding with H₂, He and Kr gas targets was measured. We obtained total cross sections and angular distributions of the charged collision fragments for the main reaction channels. H ₃ ⁺ +H₂→H⁺+2H² and H ₃ ⁺ +H₂→H ₂ ⁺ +H₂+H. An analysis of the kinetics yields that the dissociation proceeds via vibrational-rotational excitation of H ₃ ⁺ by mutually induced dipolmoments.