Gas-phase Ignition Research Articles

Ignition Behavior of Bio-Coke (Highly Densified Biomass Fuel) in High-Temperature Air Flows

The ignition behavior of a newly developed biomass briquette, Bio-coke (BIC), is investigated. The fuel has unique features such as economical advantages for its versatility of biomass resources, high volumetric calorific value because of its high density (1300 kg/m3; twice or more than that of ordinary wood pellets) and high mechanical strength. The ignition characteristics of cylindrical BIC blocks (48 mm in diameter and 85 mm in length), important when using the fuel in actual combustion furnaces, are investigated in high temperature air flows (473-873 K, 550-750 NL/min.). In the experiments, preheated air is blown onto the bottom surface of BIC cylinders and the ignition behavior of the bottom surface is observed monitoring the surface temperature as well as the time dependent mass loss rates. The results show two ignition modes; (1) solid surface ignition preceding gas-phase ignition in high air temperature conditions (T≥598K), and (2) gas-phase ignition accompanied by simultaneous surface ignition occurring at relatively low air temperature conditions. The appearance of each mode depends on the preheated air supply condition in terms of the air temperature, flow velocity, and moisture content of the fuel. The rate of evolution of volatile gases is closely correlated with the temperature distribution inside the BIC briquette which depends on the heating rate, implying that variations in the temperature distribution inside the fuel could be one reason for the appearance of the observed ignition modes. It is suggested that the temperature distribution inside the fuel has to be taken into account in the control of the ignition behavior of BIC briquettes.

Experimental and numerical investigation of hetero-/homogeneous combustion of CO/H 2/O 2/N 2 mixtures over platinum at pressures up to 5 bar

The hetero-/homogeneous combustion of fuel-lean CO/H 2/O 2/N 2 mixtures over platinum is investigated at pressures up to 5 bar, inlet temperatures ( T IN) up to 874 K, and a constant CO:H 2 molar ratio of 2:1. Experiments are performed in an optically accessible channel-flow catalytic reactor and involve planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition and 1-D Raman measurements of major gas-phase species concentrations over the catalyst boundary layer for the evaluation of the heterogeneous (catalytic) processes. Simulations are carried out with an elliptic 2-D model that includes detailed heterogeneous and homogeneous chemical reaction schemes. The predictions reproduce the Raman-measured catalytic CO and H 2 consumption, and it is further shown that for wall temperatures in the range 975 ⩽ T w ⩽ 1165 K the heterogeneous pathways of CO and H 2 are largely decoupled. However, for wall temperatures below a limiting value of 710–720 K and for the range of pressures and mixture preheats investigated, CO(s) blockage of the surface inhibits the catalytic conversion of both fuel components. The homogeneous ignition distance is well-reproduced by the model for T IN > 426 K, but it is modestly overpredicted at lower T IN. Possible reasons for these modest differences can be the values of third body efficiencies in the gas-phase reaction mechanism. The sensitivity of homogeneous ignition distance on the catalytic reactions is weak, while the H 2/O 2 subset of the CO/H 2/O 2 gaseous reaction mechanism controls the onset of homogeneous ignition. Pure hydrogen hetero-/homogeneous combustion results in flames established very close to the catalytic walls. However, in the presence of CO the gaseous combustion of hydrogen extends well-inside the channel core, thus allowing homogeneous consumption of H 2 at considerably shorter reactor lengths. Finally, implications of the above findings for the design of syngas-based catalytic reactors for power generation systems are discussed.

Ignition propensity of hydrogen/air mixtures impinging on a platinum stagnation surface

An experimental investigation into the ignition characteristics of lean pre-mixed hydrogen/air mixtures is conducted using a stagnation-point flow configuration against a platinum surface, with a special emphasis on the determination of potential fire safety hazards associated with hydrogen release in the presence of a catalyst. Two distinct regimes are observed for this system – catalytic surface reactions and gas-phase ignition. It is demonstrated that depending on mixture equivalence ratio, catalytic surface reactions can be initiated with or without surface heating. When significant surface heat is released via catalytic reactions, gas-phase ignition can be induced, greatly increasing the apparent danger of hydrogen leaks in the presence of a platinum surface. The critical surface temperatures leading to catalytic ignition for hydrogen/air mixtures over a platinum surface are further investigated over a range of equivalence ratios and stretch rates. It is shown that ultra-lean hydrogen/air mixtures can be catalytically ignited even in the absence of external heat addition, suggesting that hydrogen leakage in the presence of a platinum surface may pose a fire safety risk even at room temperature. Furthermore, even without a transition to gas-phase ignition, the surface temperature that can be sustained with surface reactions alone may contribute to component degradation or itself pose a safety hazard.

Numerical analysis of heat-mass transfer mechanisms in gas-phase ignition of films of liquid condensed substances by a laser beam

A complex of interrelated processes of heat-mass transfer with phase transitions at the ignition of liquid condensed substance by a low-power laser beam is investigated. Mechanisms of the heat-mass transfer processes are analyzed. The scale of influence of the power and the lasing range radius of the laser beam on the ignition characteristics is found. Critical conditions that do not enable the ignition in a system of liquid condensed substance film-laser beam-gas-vapor mixture are determined.

Effect of the shape of a particle heated to a high temperature on the gas-phase ignition of a liquid film

The process of gas-phase ignition of a liquid fuel film with incandescent small metal particles in the form of a parallelepiped, disk, or hemisphere was numerically simulated. The magnitude of influence of the particle shape on the delay time of ignition of a liquid fuel was determined. The range of parameters of the particle at which the effect of its shape on the ignition delay time is unimportant was established.

Effects of hydrogen preconversion on the homogeneous ignition of fuel-lean H 2/O 2/N 2/CO 2 mixtures over platinum at moderate pressures

The impact of fractional hydrogen preconversion on the subsequent homogeneous ignition characteristics of fuel-lean (equivalence ratio φ = 0.30) H 2/O 2/N 2/CO 2 mixtures over platinum was investigated experimentally and numerically at pressures of 1, 5 and 8 bar. Experiments were performed in an optically accessible channel-flow reactor and involved Raman measurements of major species over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical. Simulations were carried out with a 2-D elliptic code that included detailed hetero-/homogeneous chemistry. The predictions reproduced the LIF-measured onset of homogeneous ignition and the Raman-measured transport-limited catalytic hydrogen consumption. For 0% preconversion and wall temperatures in the range 900 K ⩽ T w ⩽ 1100 K, homogeneous ignition was largely suppressed for p ⩾ 5 bar due to the combined effects of intrinsic gas-phase hydrogen kinetics and the competition between the catalytic and gas-phase pathways for fuel consumption. A moderate increase of preconversion to 30% restored homogeneous combustion for p ⩾ 5 bar, despite the fact that the water formed due to the upstream preconversion inhibited homogeneous ignition. The catalytically-produced water inhibited gas-phase combustion, particularly at higher pressures, and this kinetic inhibition was exacerbated by the diffusional imbalance of hydrogen that led to over-stoichiometric amounts of water in the near-wall hot ignitable regions. Radical adsorption/desorption reactions hindered the onset of homogeneous ignition and this effect was more pronounced at 1 bar. On the other hand, over the post-ignition reactor length, radical adsorption/desorption reactions significantly suppressed gas-phase combustion at 5 and 8 bar while their impact at 1 bar was weaker. By increasing hydrogen preconversion, the attained superadiabatic surface temperatures could be effectively suppressed. An inverse catalytically stabilized thermal combustion (CST) concept has been proposed, with gas-phase ignition achieved in an upstream porous burner via radiative and heat conduction feedback from a follow-up catalytic reactor. This arrangement moderated the superadiabatic surface temperatures and required modest or no preheat of the incoming mixture.

The influence of radiation heat exchange on characteristics of liquid fuel ignition by a heated metal particle

A complex of interrelated heat-mass transfer processes at gas-phase ignition of a typical liquid fuel by a hot metal particle immersed partially into a liquid is investigated numerically. The scale of influence of the radiation heat exchange at particle—liquid fuel and particle—gas—vapor mixture interfaces is found. Conditions under which the impact of this factor can be neglected are determined.

Transient heat and mass transfer at the ignition of vapor and gas mixture by a moving hot particle

Abstract The processes of heat and mass transfer at the ignition of air and typical combustible liquid vapors mixture by a moving single metal particle heated till high temperatures are numerically investigated. The researches are carried out on the base of gas phase ignition model considering processes of heat conductivity, liquid evaporation, diffusion and convection of fuel vapors in oxidizing medium, ignition source crystallization, kinetic of evaporation and ignition processes, dependences of thermo physical properties of interacting substances from temperature, air humidity, heating source movement in vapor and gas mixture. The dependences of main characteristic of investigated process – ignition time delay from rate of particle motion, air humidity and temperature, distance between heating source and liquid surface, initial temperature and sizes of particle are determined.

Gas-phase ignition of a film of liquid condensed substance by a metal particle heated to high temperatures under mixed-convection conditions

A numerical analysis of the scale of influence of the mixed convection of an oxidant and a fuel vapor on the characteristics of the gas-phase ignition of the film of a typical liquid condensed substance — kerosene — by a small-size metal particle heated to high temperatures has been made. The limiting values of the velocity of air-mass motion for which no ignition conditions are realized in the system “metal particle heated to high temperatures-kerosene films-air” have been established. The ranges of variation in the air temperature and the initial warmed-up-particle temperature in which the ignition conditions are realized have been determined. It has been shown that no ignition occurs even for a high heat content of the heating source when Re > 1.3·10 3 .

Distinctive features of the gas-phase ignition of a mixture of a kerosene vapor and air by a steel wire heated to high temperatures

The process of ignition of a mixture of air and a kerosene vapor by a steel wire heated to high temperatures has been modeled numerically. The investigations have been carried out on a model allowing for the combination of heat- and mass-transfer processes (evaporation of the flammable liquid, diffusion and convection of the fuel vapor in air, heat conduction, and oxidation of the fuel vapor in air). The values of the delay time of ignition of the vapor-gas mixture have been determined and the scale of influence of the initial temperature and dimensions of the wire, the distance between the heating source and the kerosene surface, and the temperature and humidity of air on the inertia of the ignition process have been established.

On peculiarities of heat and mass transfer in a hot metal particle-liquid fuel condensed substance-air system

Results of numerical modeling are used for validating the governing role of the ratio of areas of contact between the heater with the combustible liquid and the formed vapor-gas mixture in a complex of interrelated processes of heat and mass transfer at gas-phase ignition of a liquid condensed substance film by small-size hot metal particle. Critical values of the ratio at which the ignition conditions cannot be realized are marked out. Ranges of varying the main igniter parameters for which the influence of the parameter on the ignition parameters may be neglected are determined.

The influence of heat transfer conditions at the hot particle-liquid fuel interface on the ignition characteristics

The problem of ignition in the conditions of nonideal contact between liquid fuel and a single metallic particle heated to high temperatures is numerically solved. A gas-phase ignition model is created with regard to the heat-and-mass transfer processes in the gas region near the ignition source and the layer separating the particle and the fuel. The scale of the impact of the heat source surface roughness upon the ignition characteristics in a hot particle-liquid fuel-oxidant system is determined.

3D problem of heat and mass transfer at the ignition of a combustible liquid by a heated metal particle

A nonlinear nonstationary 3D problem of heat and mass transfer at gas phase ignition of a combustible liquid spread on the surface of a solid body by a metal particle heated to a high temperature is solved. This is done within the framework of a model taking into account the heat conduction and evaporation of the liquid, the diffusion and convection of the combustible vapors in the oxidizer medium, the crystallization of the ignition source, the kinetics of the processes of evaporation and ignition of liquids, the dependence of the thermophysical characteristics of the interacting substances on the temperature, and the moisture content of the oxidizer—air. The dependences of the ignition delay time of the liquid on the temperature and sizes of the heating source are established. Limiting values of the temperature and particle sizes at which the ignition conditions take place are determined. The influence of the air humidity on the inertia of the process being investigated is analyzed. A comparison of numerical values of typical parameters of the process under investigation for 2D and 3D models is performed.

Steady-state chemiluminescence of Eu-doped yttrium oxide crystal phosphors in the catalytic reaction of hydrogen oxidation

We report on the luminescence of Eu 3+-doped yttrium oxide phosphors brought in contact with oxyhydrogen gas mixtures at temperatures much below the gas-phase ignition point. Chemical energy released in the catalytic hydrogen oxidation is passed directly to the electron subsystem of the powder and subsequently converted into the visible luminescence radiation at 2 × 10 −6 quantum yield per H 2O molecule produced. Luminescence kinetics at various conditions are investigated to reveal the constituent stages of the catalytic energy conversion process. This observation could have far-reaching consequences including possibilities for the development of original chemiluminescent light sources, and novel electrochemical energy converters.

Hetero-/homogeneous combustion of hydrogen/air mixtures over platinum at pressures up to 10 bar

The hetero-/homogeneous combustion of fuel-lean hydrogen/air premixtures over platinum was investigated experimentally and numerically in the pressure range 1 bar ⩽ p ⩽ 10 bar. Experiments were carried out in an optically accessible channel-flow catalytic reactor and included planar laser induced fluorescence (LIF) of the OH radical for the assessment of homogeneous (gas-phase) ignition, and 1-D Raman measurements of major gas-phase species concentrations for the evaluation of the heterogeneous (catalytic) processes. Simulations were performed with a full-elliptic 2-D model that included detailed heterogeneous and homogeneous chemical reaction schemes. The predictions reproduced the measured catalytic hydrogen consumption, the onset of homogeneous ignition at pressures of up to 3 bar and the diminishing gas-phase combustion at p ⩾ 4 bar. The suppression of gaseous combustion at elevated pressures bears the combined effects of the intrinsic homogeneous hydrogen kinetics and of the hetero/homogeneous chemistry coupling via the catalytically produced water over the gaseous induction zone. Transport effects, associated with the large diffusivity of hydrogen, have a smaller impact on the limiting pressure above which gaseous combustion is suppressed. It is shown that for practical reactor geometrical confinements, homogeneous combustion is still largely suppressed at p ⩾ 4 bar even for inlet and wall temperatures as high as 723 and 1250 K, respectively. The lack of appreciable gaseous combustion at elevated pressures is of concern for the reactor thermal management since homogeneous combustion moderates the superadiabatic surface temperatures attained during the heterogeneous combustion of hydrogen.

The role of secondary char oxidation in the transition from smoldering to flaming

The transition from forward smoldering to flaming in polyurethane foam is observed using in-depth thermocouples and ultrasound probing. The experiments are conducted with small parallelepiped samples vertically placed in an upward wind tunnel. Three of the vertical sample sides are maintained at elevated temperature and the fourth is exposed to an upward oxidizer flow and a radiant heat flux. An ultrasound probing technique is used to measure the line-of-sight average permeability of the sample at the same heights as the thermocouples. The smolder front propagation is tracked by both the thermocouples and ultrasound data, which show an increase in temperature and permeability upon passage of the smolder front. The permeability data also show that the transition to flaming is preceded by rapid fluctuations in permeability in the char region below the smolder front, indicating the formation of pores by secondary char oxidation. The pores provide locations for the onset of gas-phase ignition (i.e., transition to flaming). The results from all the tests indicate that the formation of pores is a necessary but not sufficient condition for the transition to flaming. Two novel measures of the intensity of the secondary char oxidation are introduced: the time derivative of permeability, and the secondary char oxidation velocity. The time derivative of permeability, which provides a measure of the pore formation rate, is found to increase as the oxygen concentration and/or radiant heat flux increase, and to indicate the likelihood of the transition to flaming. The permeability data offers a means to track the propagation of the secondary char oxidation, and to calculate the secondary char oxidation velocity, which is found to be strongly correlated to the transition to flaming. A simplified energy balance model is able to predict the dependence of the secondary char oxidation velocity on oxygen concentration and radiant heat flux.

Laser induced fluorescence of formaldehyde and Raman measurements of major species during partial catalytic oxidation of methane with large H 2O and CO 2 dilution at pressures up to 10 bar

The catalytic partial oxidation (CPO) of CH 4/O 2 mixtures diluted with large amounts of H 2O and CO 2 (up to 43% and 21% vol., respectively) was investigated experimentally and numerically in the pressure range 4 bar ⩽ p ⩽ 10 bar. Experiments were carried out in an optically accessible channel-flow catalytic reactor coated with Rh/ZrO 2, and included planar laser induced fluorescence (LIF) of formaldehyde for the assessment of homogeneous (gas-phase) ignition and one-dimensional spontaneous Raman measurements of all major gas-phase species for the evaluation of the heterogeneous (catalytic) processes. Simulations were performed with a full elliptic model that included detailed heterogeneous and homogeneous chemical reaction schemes. Over the reactor length with negligible gas-phase chemistry contribution, the employed heterogeneous reaction scheme provided good agreement to the measured methane consumption and synthesis gas yields, overpredicting mildly the partial over the total oxidation route. It was shown that the added water provided a source of O(s) and OH(s) surface radicals that enhanced the methane conversion and H 2 yields and reduced the CO yields. Moreover, the addition of CO 2 had a negligible chemical effect on the aforementioned parameters. An increase in pressure from 4 to 10 bar had a minor impact on the methane conversion and hydrogen selectivity. The employed gaseous scheme reproduced the LIF-measured onset of homogeneous ignition, although it underpredicted the extent of the formaldehyde zone ahead of the flame and the flame propagation characteristics at the highest investigated pressure (10 bar).

Laser Ignition and Combustion Properties of Composite Propellant Containing Nanometal Powders

The effect of nanometal powders (nano-grain-size aluminum powders and nanograin size nickel powders) on the ignition delay time of ammonium perchlorate composite propellants has been examined in detail within a carbon dioxide laser ignition system. A comparison of different experimental techniques for combustion-related properties evaluation, including burning rates, deflagration heat, and ignition temperatures of the ammonium perchlorate propellants containing nanometal powders is presented. A thermogravimetric analyzer is employed to investigate the thermal behavior of nano-grain-size and general-grain-size aluminum powders heated in air. The content of active aluminum in the combustion residue was measured by means of the titration. The results show that, with the cooperating effect of nanonickel powders, the addition of nanoaluminum powders in propellants can greatly increase the burning rate, shorten the ignition delay time, and improve the combustion efficiency of aluminum in the combustion of propellant. The individual application of nanometal powders (nanoaluminum or -nickel powders) displayed an intermediate effect, whereas the general-grain-size nickel powders had the least effect. The effect of nano-grain-size Al is determined to be on the solid-phase ignition of the propellant. The effect of nano-grain-size Ni is determined to be on the gas-phase ignition of propellant initially and the solid-phase ignition subsequently.

Ignition of Combustion Modified Polyurethane Foam

Abstract Results are presented from an experimental study on the ignition of the combustion modified (fire retarded) polyurethane foam Pyrell® (35.3 kg/m3 and 64.0 kg/m3) in elevated oxygen concentrations, ranging from 30 % to 60 %. The samples are exposed to an external flow and variable radiant heat flux on one face, and insulated on the other faces. The experiments show that Pyrell® undergoes a weak smoldering reaction that requires significant assistance in the form of external heat input in order to propagate. The results also show that given sufficient oxygen and radiant heat flux, the smoldering reaction can produce enough volatile fuel and heat to trigger a gas-phase ignition, i.e. a transition from smoldering to flaming, in pores in the char region. The experiments also indicate that high-density Pyrell® is more ignitable than low-density Pyrell®, which could be explained by the greater solid surface area for smoldering reactions to take place.

Temperature profiles and residence time effects during catalytic partial oxidation and oxidative steam reforming of propane in metallic microchannel reactors

Microchanneled reactors made from Fecralloy and impregnated with Rh were studied in partial oxidation (POX) and oxidative steam reforming (OSR) reactions for production of hydrogen or synthesis gas. Temperature profiles obtained along the reactor axis confirmed that the metallic microchannel reactor is able to minimize temperature gradients and pointed to gas phase ignition in front of the reactor as the main reason for reduced syngas production and increased hydrocarbon by-product formation at the highest furnace temperatures. Addition of steam to the reactant mixture (OSR) resulted in increased formation of hydrogen relative to propane converted (S*(H 2)), mainly in accordance with the water–gas shift reaction. Reducing the residence time below 10 ms at 800 °C gave increased S*(H 2) and CO selectivity and reduced by-product formation, indicating suppression of the gas phase reactions and possibly also direct formation of H 2 and CO as part of the reaction scheme. The Rh/Al 2O 3/Fecralloy microchannel reactor also shows good stability during more than 70 experiments of both POX and OSR with temperature cycling from 300 to 1000 °C.