H2 Combustion Research Articles

Two Reaction Paths at Different Temperatures in the Reduction of Nitrogen Monoxide with Hydrogen over Supported Palladium Catalysts

Abstract Two conversion maxima at 373K and 573K have been found to appear in the NO reduction with H2 over metal oxides supporting Pd, when O2 is present in the stream of reactants. At the lower temperature, NO directly reacts with H2, but primarily produced NO2 on the catalyst can successively react with H2 in preference to the simple combustion of H2 at the higher temperature.

The Gas Combustion of H2 with N2O Used for Rapid Thermal Annealing

The gas combustion of H2 with N2O was investigated to develop a rapid thermal annealing method. Spectra of the light emission caused by the combustion show a gray body irradiation characteristic with a gas temperature of 2200 K with an initial total gas pressure of 500 Torr ([H2]/[N2O]=1). A transient thermometry with a 100-nm-thick Cr film as a temperature sensor formed on a quartz substrate was used to measure temperature changes at the surface during and after combustion. Heating to 800°C was achieved within 2 ms using a substrate preheated to 300°C. The increase of electrical conductivity was achieved from 4×10-7 S/cm to 7×10-4 S/cm by the combustion for 0.5% phosphorus-doped amorphous silicon films.

Observations of H2O formation in the supersonic combustion of H2 by high-enthalpy air

We have investigated and compared the dynamics of supersonic H2-air combustion for compression and expansion swept-ramp, three-hole injector configurations in a laboratory shock tunnel. Hydrogen fuel was injected into a Mach 3 flow of 1200 K air at 0.67 atm. The experiments used a combination of imaging and line-of-sight optical diagnostics at different locations downstream of the point of injection. Images of OH(A2Σ) and OH(X2II) spatial distributions provide information on the near-field mixing and ignition characteristics and far-field vertical spreading of the reacting layer, while spectral measurements of H2O(v2) emission provide the far-field temperatures and H2O yields in the reacting layer. The two injector configurations display markedly different ignition characteristics but similar near-field air-fuel mixing behavior and far-field product yields. The far-field measurements at long convective flow times give final H2O concentrations, temperatures, and OH(A) concentrations that are greatest near the bottom wall and decrease with increasing height. H2O and OH(A) are observed only in the lower half of the duct; thus complete mixing of the fuel with the main flow is not achieved. For both injectors, the efficiency of H2O production compared to full equilibrium for an idealized premixed system is 30–40%. Kinetic limitations due to incomplete radical removal are relatively minor. The primary factor limiting the far-field combustion efficiency appears to be poor mixing due to the slow vertical propagation of the reacting layer, which is similar for the two injectors despite the differences in their near-field ignition behavior.

Kinetics of combustion of H2+O2 mixture with participation of vibrationally excited molecules

We analyze the laws governing the variation in chemical reaction rates in a reacting H2+O2 gas mixture with preliminarily excited vibrational degrees of freedom of H2 or O2 molecules. It is shown that with participation of vibrationally excited H2 and O2 molecules in the combustion process, a substantial decrease in the induction period and combustion time is possible.

Chemistry in plumes of high-flying aircraft with H2 combustion engines: a modelling study

Abstract. Recent discussions on high-speed civil transport (HSCT) systems have renewed the interest in the chemistry of supersonic-aircraft plumes. The engines of these aircraft emit large concentrations of radicals like O, H, OH, and NO. In order to study the effect of these species on the composition of the atmosphere, the detailed chemistry of an expanding and cooling plume is examined for different expansion models. For a representative flight at 26 km the computed trace gas concentrations do not differ significantly for different models of the expansion behaviour. However, it is shown that the distributions predicted by all these models differ significantly from those adopted in conventional meso-scale and global models in which the plume chemistry is not treated in detail. This applies in particular to the reservoir species HONO and H2O2.

Experimental investigation of H2 combustion and supersonic cooling in an annular channel

Results are presented for an experimental investigation of H2 combustion and cooling in a channel which simulates the intake and combustion chamber of a supersonic ram jet. Stable operation conditions were obtained for operating the intake and combustion chamber with various fuel distributions through the feed zones. The cooling rate and the combustion efficiency were determined at the chamber wall at varying flows in the channel.

Unburned Hydrocarbons in a Spark-ignition Engine Cylinder

C3H8-O2-N2 mixtures were spark-ignited in a motored engine cylinder, and the mass fractions of CO2, CO, H2 and hydrocarbons in the cylinder during the exhaust process were investigated. In the vicinity of the limits of flame propagation in the engine cylinder, branched flames, which propagate at relatively slow speed, appear. In the mixture composition in which the branched flames appear, the mass fraction of CH4 increases extremely, but no discontinuous changes occur in the mass fraction of CO2, CO, O2, H2, and high combustion efficiency was recognized. Furthermore, the effects of pressure and temperature on the quenching distance were expressed by an empirical equation. Quenching distance at combustion end were calculated, and the change in the mass fraction of total hydrocarbons versus the mixture composition was estimated.

Unburned Hydrocarbons in a Spark-Ignition Engine Cylinder

C3H8-O2-N2 mixtures were spark-ignited in a motored engine cylinder, and the mass fractions of CO2, CO, H2 and hydrocarbons in the cylinder during the exhaust process were investigated. In the vicinity of the limits of flame propagation in the engine cylinder, branched flames, which propagate at relatively slow speed, appear. In the mixture composition in which the branched flames appear, the mass fraction of CH4 increases extremely, but no discontinuous changes occur in the mass fraction of CO2, CO, O2, H2, and high combustion efficiency was recognized. Furthermore, the effects of pressure and temperature on the quenching distance were expressed by an empirical equation. Quenching distance at combustion end were calculated, and the change in the mass fraction of total hydrocarbons versus the mixture composition was estimated.

Choice of optimum parameters in stabilizing combustion in a flow with a forechamber jet of H2 combustion products

In this paper, the authors examine the effects of the basic parameters in a jet of combustion products from hydrogen-air mixtures as regards the concentration limits for the stabilization of the flame from previously mixed propane-air mixtures in a turbulent flow. Also determined were the simulation criteria for the stabilization with the use of forechamber devices. The results show that the stabilization effect is greatest when the excess air coefficient in the forechamber approx. = 0.7. Results indicate that chemical and aerodynamic factors have important effects in the forechamber method of stabilizing combustion in a flow. Theoretical analysis of the stabilization conditions is based on an ignition model for a turbulent flow containing a hot gas jet. In the simulation, it is assumed that ignition by the forechamber method is due to the temperature rise in the flow of the main mixture because the hot combustion products from the forechamber mix in with it. Calculations of the concentrations show that there is a logarithmic relationship for the effect of the initial activecenter concentration on the ignition for isothermal conditions. The active-center concentration is not quantitatively increased by enriching the mixture with fuel, nor is the chemical activity in the flamemore » raised. One can explain results in which there was some improvement in the performance of a forechamber device when the forechamber mixture was rich by saying that there was an increased concentration of molecular hydrogen in the forechamber flame.« less

Erta'ale lava lake: heat and gas transfer to the atmosphere

Abstract Data on uncontaminated samples of volcanic gases can be counted on the fingers of one hand, yet estimation of total volcanic gas flow cannot be made without such data. In this paper the flux of gas from the lava lake to the atmosphere is calculated by a heat budget based on the excess heat loss caused by combustion of H2 and CO and by the mass rate of loss of other gases on the basis of their ratios to H2 and CO in the unoxidized gas samples. The estimated rates of loss of H2O, CO2, SO2 and HCl are consistent with the rate of loss of heat if this heat is generated by crystallization and if the initial magma contains concentrations of gas appropriate for submarine basalt from oceanic ridges. The moderate activity of permanent degassing from the two active lava ponds studied gives a lower flux than that of other volcanoes.

The role of heterogeneous factors and heat in the combustion of hydrogen

1. In the combustion of H2, “anomalously” profound conversions are observed not only under conditions when chain termination occurs primarily only by sorption of H atoms on the wall, but also under conditions when the sorption of O atoms and OH radicals is also significant. 2. The role of spontaneous heating of the investigated reaction mixture is inconsequential.

Rate constant for H+H+Ar = H2 + Ar from 1300 to 1700 K

AbstractShock tube experiments on the decay of OH‐radical concentration after shock‐initiated combustion of H2:O2:Ar = 10:1:89 mixtures were analyzed to give the rate constant 1 × 1015 cm6mol−2s−1for the reaction H + H + Ar = H2 + Ar overthe temperature range 1300 to 1700 K.

Study of the sensitivity of coupled reaction systems to uncertainties in rate coefficients. II Applications

The Fourier amplitude method developed in Paper I as a diagnostic tool for determining the sensitivity of the results of complex calculations to the parameters which enter these calculations has been applied to two chemical reaction systems involving sets of coupled, nonlinear rate equations. These were: (a) a five reaction set describing the high temperature (6000 °K) dissociation of air and (b) a nine reaction set describing the constant temperature (2000 °K) combustion of H2 and O2. We have evaluated the Fourier amplitudes for all the species at a number of different times for both reaction systems. The analysis of these results verifies the claims made in Paper I. The relative magnitudes of the Fourier amplitudes showed a several order of magnitude distribution which permitted a clear distinction of the relative sensitivity of the species concentration to uncertainties in the rate coefficients. The conclusions based on the Fourier amplitude method for these two reaction systems are in excellent agreement with sensitivity predictions which could be made on the basis of previous kinetic studies of these systems.

HO 2 radicals in combustion reactions

D discussion of the role of HO2 radicals in combustion reactions is presented. It is shown thatelementary reactions of HO2 with H atoms to form H2O+O, H2+O2, and OH are of importance, and should not be neglected in the mechanism of H2 oxidation and combustion, particularly near the explosion limits. Depending upon the flame conditions, HO2 can behave as a chain terminator or propagator. Analysis of data on the effect of CO on H2/O2 explosion limits and H2O2 decompositionsuggests that the rate of the reaction HO2+CO=CO2OH would be too slow to play an important role in CO combustion.

Kinetic Studies of Hydroxyl Radicals in Shock Waves. II. Induction Times in the Hydrogen-Oxygen Reaction

The formation of OH in the shock wave induced combustion of H2 and O2 has been measured by oscillographically recording the absorption of ultraviolet OH line radiation. The main features of the reaction course are: (1) an induction period whose length, ti, varies inversely with [O2], (2) an increase in the product [O2] ti as ti becomes short compared to the vibrational relaxation time of O2, and (3) at the end of the induction period, a sigmoid rise of [OH] to a maximum, followed by a slow decrease. ti has been studied over the ranges: 1100°≤T≤2600°K, 1.3×10—5≤[O2]≤8.0×10—4 mole/1, 0.25≤[H2]/[O2]≤5., 0.004≤[O2]/[Ar]≤0.20, and 5≤ti≤500 μsec. Agreement between incident and reflected shock experiments has been demonstrated. According to the branching chain mechanism known from explosion limit studies, ti is governed by the rate of H+O2→ lim k1OH+O according to: 2 k1[O2]ti=2.303 n, where n is the number of decades by which [OH] increases between initiation and the end of the induction period. The values of [O2]ti, which is nearly proportional to 1/k1, are summarized by: log10([O2]ti) (mole 1—1 sec)= —10.647+(3966±625)/T. The value k1=1.4×109 deduced at 1650°K from this work is combined with data near 800°K to give: k1=3×1011 exp(—17.5±3. kcal/RT) (mole/1)—1 sec.—1. The relation of these results to detonation experiments is discussed.

Double cylinder expansion marine engines

Hydrogen (H2) is currently a highly attractive fuel for internal combustion engines (ICEs) owing to the prospects of potentially near-zero emissions. However, the production emissions and cost of H2 fuel necessitate substantial improvements in ICE thermal efficiency. This work aims to investigate a potential implementation of H2 combustion in a highly efficient double compression-expansion engine (DCEE). DICI nonpremixed H2 combustion mode is used for its superior characteristics, as concluded in previous studies. The analysis is performed using a 1D GT-Power software package, where different variants of the DICI H2 and diesel combustion cycles, obtained experimentally and numerically (3D CFD) are imposed in the combustion cylinder of the DCEE. The results show that the low jet momentum, free jet mixing dominated variants of the DICI H2 combustion concept are preferred, owing to the lower heat transfer losses and relaxed requirements on the fuel injection system. Insulation of the expander and removal of the intercooling improve the engine efficiency by 1.3 and 0.5%-points, respectively, but the latter leads to elevated temperatures in the high-pressure tank, which makes the selection of its materials harder but allows the use of cheaper oxidation catalysts. The results also show that the DCEE performance is insensitive to combustion cylinder temperatures, making it potentially suitable for other high-octane fuels, such as methane, methanol, ammonia, etc. Finally, a brake thermal efficiency of 56% is achieved with H2 combustion, around 1%-point higher than with diesel. Further efficiency improvements are also possible with a fully optimized H2 combustion system.