Decrease In Ignition Temperature Research Articles

Numerical investigations on hydrothermal flame characteristics of water-cooled hydrothermal burner

Abstract Supercritical hydrothermal combustion, as a quick homogeneous oxidizing process, offers a promising treatment option for industrial wastewater. This paper established a computational fluid dynamics model of a water-cooled hydrothermal combustion burner to investigate the thermal flame characteristics. The effects of the fuel mass flow rate, fuel concentration, initial reactor temperature, reaction pressure, and oxidant temperature on the thermal combustion ignition were revealed. The results indicate that the fuel concentration (from 10 wt% to 60 wt%) and initial reactor temperature (from 623 to 773 K) had less effect on the ignition temperature. In contrast, the ignition temperature increases by 398 K with increasing fuel mass flow rate (from 24 kg h−1 to 1080 kg h−1). As the oxygen temperature increases (from 273 to 673 K), the ignition temperature gradually decreases to 573 K and then increases. An increase in reaction pressure can facilitate a decrease in ignition temperature to a certain extent, and the optimal reaction pressure is 25 MPa. This study provides a vital reference for a hydrothermal burner’s scale-up design and ignition operation.

The ignition of fine iron particles in the Knudsen transition regime

A theoretical model is considered to predict the minimum ambient gas temperature at which fine iron particles can undergo thermal runaway–the ignition temperature. The model accounts for Knudsen transition transport effects, which become significant when the particle size is comparable to, or smaller than, the molecular mean free path of the surrounding gas. Two kinetic models for the high-temperature solid-phase oxidation of iron are analyzed. The first model (parabolic kinetics) considers the inhibiting effect of the iron oxide layers at the particle surface on the kinetic rate of oxidation, and a kinetic rate independent of the gaseous oxidizer concentration. The ignition temperature is solved as a function of particle size and initial oxide layer thickness with an unsteady analysis considering the growth of the oxide layers. In the free-molecular limit (small particles), the thermal insulating effect of transition heat transport can lead to a decrease of ignition temperature with decreasing particle size; however, the presence of the oxide layer slows the reaction kinetics, and its increasing proportion in the small-particle limit can lead to an increase of ignition temperature with decreasing particle size. This effect is observed for sufficiently large initial oxide layer thicknesses. The continuum transport model is shown to predict the ignition temperature of iron particles exceeding an initial diameter of 30 µm to a difference of 3% or less (30 K or less) when compared to the prediction of the transition transport model. The second kinetic model (first-order kinetics) considers a porous, non-hindering oxide layer, and a linear dependence of the kinetic rate of oxidation on the gaseous oxidizer concentration. The ignition temperature is resolved as a function of particle size with the transition and continuum transport models, and the differences between the ignition characteristics predicted by the two kinetic models are identified and discussed.

The effect of cellulose on the combustion characteristics of coal slime: TG-FTIR, principal component analysis, and 2D-COS

Facing the severe environmental and energy crisis, how to use solid waste cleanly and efficiently is imminent. In this study, coal slime and cellulose are used for co-combustion experiments. Coal slime and cellulose are tested at mass ratios of 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10. When 10 % cellulose is added to the coal slime, the ignition temperature decreases by 169.3 °C, and the ignition characteristic index increased by 2.9 times. The addition of cellulose makes the comprehensive combustion performance of the sample increase exponentially. By analyzing the dominant reaction in the co-combustion process of coal slime and cellulose, the variance contribution rates of the first principal component and the second principal component are 67.84 and 29.18 %, respectively. Co-combustion of coal slime and cellulose enhances the release and combustion of early volatiles. In addition, the evolution law of gaseous products and the temperature dependence of gaseous products in the co-combustion process of coal slime and cellulose are analyzed by fourier transform infrared spectroscopy and two-dimensional correlation spectroscopy, respectively. This study provides a theoretical basis for the large-scale application of co-combustion of coal slime and biomass.

Effect of interactions during co-combustion of organic hazardous wastes on thermal characteristics, kinetics, and pollutant emissions

Thermal disposal of organic hazardous wastes (OHWs) in a rotary kiln is an effective method to destroy organic pollutants and reduce the volume, but the complex interactions between various OHWs may result in sharp degradation on combustion performance and the increase in gaseous pollutants emission. Herein, three typical types of OHWs (i.e., pesticide waste, dyeing waste, and organic resin waste, labeled as HW1, HW2, and HW3, respectively) were chosen and thermally co-treated, and the co-combustion characteristics, kinetics, and gas evolutions were systematically studied. A strong positive interaction between HW1 and HW2 was found between 440 and 680 °C possibly due to the catalytic effect of Fe (III) and alkali metals in HW1. The experimental DTG peaks of mixtures at 480 °C were advanced by 60 °C compared with the calculated ones, resulting from the volatiles combustion of HW2 and the catalytic effect from Fe2O3 formed during the combustion. The decrease of ignition temperature of mixtures was found helpful for stable combustion, while the decrease of burnout temperature during co-combustion of HW2 and HW3 exhibited the potential to reduce the clinker ignition loss.

Experimental Study on Co-Firing of Coal and Brewery Wastewater Sludge

The environmental pollution and high energy consumption caused by the coal-dominated energy structure in China have been the focus of attention for a long time. The co-firing of biomass with coal can save coal resources and realize effective utilization of biomass. In this paper, brewery wastewater sludge (SD) and bituminous coal (BC) were blended for an experimental study which aimed to provide basic experimental data and operational guidance as a reference for practical application in power plants. The co-firing characteristics of sludge and bituminous coal were studied. The results show that the burnout temperature and ignition temperature decrease with an increase in the sludge blending ratio. The Comprehensive Combustion Index (CCI) first rises, then decreases, reaching a maximum at about 15%. Compared with the atmosphere with 79% N2/21% O2, under the 79% CO2/21% O2 atmosphere, ignition is delayed and the burnout temperature is higher. Under an O2/CO2 atmosphere, as the O2 concentration improves, the thermo-gravimetric (TG) curve shifts to the low-temperature region, the burnout temperature drops significantly, and the comprehensive combustion characteristics are improved. With an increment of the heating rate, the curve of TG analysis shifts to the high-temperature region and the CCI increases. This study could provide helpful information on practical blending in coal-fired power plants for energy savings and emission reductions.

Experimental investigation on ignition and combustion characteristics of n-butane/air mixtures by glow plug in miniature chamber

The aim of this work is to understand the ignition and combustion behaviors of n-butane/air mixtures under glow plug ignition in the start-up condition of miniature IC engines. We utilized a centimeter-scale constant-volume chamber setup to investigate the effects of glow plug heating power, equivalence ratio, and initial pressure on the characteristics of ignition and flame propagation of n-butane/air mixtures. Results indicated that the ignition time and ignition temperature decrease as the heating power increases and the flame propagation time is weakly sensitive to that. Increasing the initial pressure can reduce the ignition time, decrease the ignition temperature and slow flame propagation; except for the flammable limit, the ignition time and ignition temperature increase as the equivalence ratio increases, in addition, the flame propagation is fastest at φ = 1.2. Furthermore, an obvious two-stage combustion phenomenon was observed under fuel-rich flame propagation conditions. Overall, the order of influencing factors on the ignition characteristics shows heating power > initial pressure > equivalence ratio, and that on the flame propagation characteristics shows equivalence ratio > initial pressure > heating power in present experimental conditions.

Some metallic nanoparticles’ ignition temperature by thermodynamic consideration

Self-propagating high-temperature synthesis (SHS) provides an attractive alternative route for the preparation of various compounds. Considering the diffusion reaction at the solid interface, the size-dependent activation energy and ignition temperature of the compounds synthesized by SHS can be used to help in modeling. Therefore, a quantitative model for size-dependent ignition temperature is established by considering energetic and physical characteristics. To the author’s knowledge, this is the first theoretical model to explore the effects of size on ignition temperature for SHS applications, with size ranging from nano- to microscale. An assumption of ideal SHS is made in the deduction of the model. As the size of reactants decreases, the ignition temperature decreases. This is because increasing the contact area and reducing the diffusion barrier between the reactants must be used specifically to calculate the reactants at the nanometer scale. The model predictions are still in good agreement with the experimental and theoretical observations for metallic aluminides and titanium carbide systems.

Effect of metal oxide nanoparticles on the ignition characteristics of diesel fuel droplets: an experimental study

The present study experimentally investigates the effect of metal oxide nanoparticles on the ignition probability, ignition zone, and burning time of the diesel fuel. For this purpose, a series of hot plate ignition tests are conducted on the diesel fuel droplets with and without nanoparticles. The comparative performances of the pure diesel and diesel fuel containing 0.03 and 0.05 wt% of Al2O3, CeO2, Fe3O4, and TiO2 nanoparticles are examined. The thermo-physical properties of the nanofuels, including thermal conductivity, viscosity, and heat capacity are also measured. The experimental results show that in the presence of metal oxide nanoparticles, the ignition probability significantly increases. It is observed that cerium oxide nanoparticles show the greatest effect on the ignition probability. In addition, it is found that the minimum hot plate ignition temperature decreases. Results show that the burning time and the ignition zone decrease by adding metal oxide nanoparticles.

Effect of hydrogen addition on the counterflow ignition of n-butanol at atmospheric and elevated pressures

The present experimental and computational study describes the non-premixed counterflow ignition of nitrogen-diluted pre-vaporized n-butanol/hydrogen mixtures impinging on a heated air stream for pressures of 1–4 atm and hydrogen molar percentages in the binary fuel blends ranging from ξH = 0% (pure n-butanol) to ξH = 100% (pure hydrogen). The experimental data show that hydrogen addition results in a non-linear decrease in ignition temperatures that can be broken into two regimes; a hydrogen-enhanced regime of ξH = 0–40%, where the addition of more hydrogen significantly decreases ignition temperature, and a hydrogen-dominated regime in the range of ξH = 40–100%, where ignition temperatures exhibit minimal sensitivity to further hydrogen addition. The experimental results are then simulated using n-butanol-specific skeletal mechanisms developed from two comprehensive butanol models available in the literature. The ability of these mechanisms to predict the variation of ignition temperatures associated with hydrogen addition to the n-butanol “base” fuel is assessed. Comparison between the experimental and computational results reveals that both chemical kinetic models capture the two-regime behavior associated with hydrogen addition observed in the present experiments, though both models over-predict experimental ignition temperatures. Further chemical kinetic analysis of the mechanisms reveals that the two-regime behavior is controlled by the production of hydroperoxyl radicals, with production via the reaction of formyl radicals and oxygen molecules dominating at low hydrogen addition levels, and production via the three-body H + O2+M = HO2 + M reaction dominating at high hydrogen addition levels.

Low-temperature catalytic oxidation of multi-walled carbon nanotubes

When in a pure form, carbon nanotubes are known to be stable in air up to ∼800K making them attractive for a large variety of applications. In this work, we report a significant decrease of ignition temperature (in some cases occurring at ∼500K) and a reduction in the apparent activation energy for oxidation in air as a result of impregnation with nanoparticles (<2nm) of metal (Pt, Pd, Ni and Co) acetylacetonates or by decoration with corresponding oxides. Surprisingly, defects introduced by partial oxidation of the carbon nanotubes do not in practice have any influence on the enhancement of further oxidation. Reduction temperatures of metal oxides with H2 were close to those of other carbon supported catalyst materials. However, the carbon nanotubes showed a tendency for low temperature gasification in the presence of hydrogenation catalyst metals (Pt, Pd).

A Thermogravimetric Study of the Behaviour of Biomass Blends During Combustion

The ignition and combustion behavior of biomass and biomass blends under typical heating conditions were investigated. Thermogravimetric analyses were performed on stalk and woody biomass, alone and blended with various additive weight ratios. The combustion process was enhanced by adding oxygen to the primary air. This led to shorter devolatilization/pyrolysis and char burnout stages, which both took place at lower temperatures than in air alone. The results of the ignition study of stalk biomass show a decrease in ignition temperature as the particle size decreases. This indicates homogeneous ignition, where the volatiles burn in the gas phase, preventing oxygen from reaching the particle surface.The behavior of biomass fuels in the burning process was analyzed, and the effects of heat production and additive type were investigated. Mixing with additives is a method for modifying biofuel and obtaining a more continuous heat release process. Differential scanning calorimetric-thermogravimetric (DSC-TGA) analysis revealed that when the additive is added to biomass, the volatilization rate is modified, the heat release is affected, and the combustion residue is reduced at the same final combustion temperature.

Modeling the effect of particle size on the activation energy and ignition temperature of metallic nanoparticles

The present work reports a simple theoretical model to calculate the effect of the particle size on the activation energy and the ignition temperature of metallic nanoparticles. The activation energy was deduced from the particle cohesive energy and the ignition temperature was calculated using the condition that the heat generated by the combustion reactions is sufficient to counterbalance the particle heat loss to the surrounding. Heat loss was assumed to be in the transient regime and the combustion heat generation was calculated using the simplest Arrhenius-type model. Using aluminum as an example, the results showed that for particles of sizes larger than 50 nm, increasing the particle size had a little effect on the number of the surface atoms, the activation energy and the ignition temperature. As the particle size decreases the number of the surface atoms increases and the corresponding activation energy, E d / E ∞ and the ignition temperature decrease. As the particle size decreased to about 5 nm and smaller, the activation energy could reduce to 20% or 50% of the bulk value and an ignition temperature as low as 800 K was obtained from the calculation depending on the ratio of the coordination number.

Investigation of co-combustion characteristics of low quality lignite coals and biomass with thermogravimetric analysis

In this study, combustion and co-combustion characteristics of three biomass fuels and three Turkish lignite coals were investigated by using a thermo gravimetric analyzer. There are just a few studies investigating the co-combustion characteristics of coal and biomass, and the synergistic effect of their various combinations on the peak temperatures and burnout times. Results of this study have shown that low rank coals can be burned with biomass very beneficially. As the VM content of the coal increases, the ignition temperature decreases and the maximum combustion rate is reached at lower temperatures. About 80–90% of the fuel is combusted as the combustion of volatile matter. As the biomass content of the fuel mixture increases, the ignition temperatures of the mixtures are found to be very close to that of the biomass. Therefore, it can be said that biomass in the mixture starts to burn first and then coal follows.

Formation mechanism of MoSi2 in presence of copper by combustion synthesis: study in thermal explosion mode

The effect of copper addition on the formation mechanism of MoSi2 through combustion synthesis (thermal explosion mode) from elemental powder mixtures was evaluated. It was proposed that in the presence of copper, the reaction starts by forming a low melting point and eutectic melt of Cu–Si, and proceeds by the dissolution of Mo in the melt. The effect of copper addition on the ignition and combustion temperatures was also investigated. It was found that by increasing the copper amount, the ignition temperature decreases but the combustion temperature increases. This effect was also attributed to the formation of low temperature eutectic melt which causes the initiation of the reaction at lower temperatures. This melt also increased the mass and heat transfer rate, which leads to a higher reaction rate and combustion temperature.

Ignition of Disperse Systems During Stage Processes on Their Surface

The possibility of moderating the conditions for ignition of a solid combustible that has a difficultly penetrable oxide coating on its surface is illustrated by solution of the problem of ignition of a gas suspension of boron particles in an oxygen—water vapor mixture as an example. The decrease in the ignition temperature of this combustible is reached owing to an active gaseous reagent capable of reacting with an oxide film and transforming it to the products which having no blocking properties. The calculation has shown that if the B2O3 film which hinders particle ignition is removed only as a result of its evaporation, the ignition temperatures of the particles are high. Gasification of the oxide significantly accelerates the removal of the film, which causes a decrease in ignition temperatures compared to a dry medium. The experiment has supported the validity of the calculation. The effect is manifested the stronger the higher the reaction surface of the reacting system. The critical condition for ignition of a gas mixture of particles is obtained from analysis of the stationary solutions of a system of equations on the stability to small perturbations. It is shown that the optimum ratio of the oxidizing components at which the minimum ignition temperature of the suspension occurs exists.

Thermal testing methods in determination of characterization of charcoals

Thermal analysis testing methods were used in determination of the characterization of charcoals. Thermogravimetry (TG) method was adopted to determine the composition of charcoals, which include moisture, volatiles, fixed carbon and ash contents. The result showed that this method could detect the subtle change of charcoal composition, even the variation of different parts of material. Differential Thermal analysis (DTA) and related methods were also used to investigate reactivity of charcoals. The ignition temperature decrease with increasing carbon content was detected by these methods.

Hot wire ignition of hydrogen-oxygen mixtures

Experiments were conduted to measure the temperature at which hot wires ignited hydrogen-oxygen mixtures. The wires served both as ignition sources and as resistance temperature sensors. Gas compositions were varied between lean and rich flammability limits (4%–94% hydrogen) and initial gas pressures from 100 to 110 kPa. A simple model of hot surface ignition used to correlate results gave reasonable predictions for the variation of ignition temperature with hydrogen concentration. However, the ignition temperature was reduced only by approximately 50 K when the pressure was raised to 1100 kPa. The model predicted a much larger decrease in ignition temperature, suggesting that the assumption of a single-step chemical reaction made in its formulation was not valid as elevated pressures.

INVESTIGATION INTO THE IGNITION PROPERTY OF COAL UNDER PRESSURE

ABSTRACT The ignition property (ignition temperature and combustion time) of coals and chars were determined at the heating rate of 7.5°/min and under pressure. Different factors have been investigated, which include particle size, total pressure, partial pressure of oxygen, macerals, preparation method and catalyst. The results show that: the ignition temperature decrease with decreasing particle size and increasing total pressure or partial pressure of oxygen; coal char prepared by lower heating rate has higher ignition temperature than that by higher heating rate; vitrinite has higher ignition temperature than fusinite, but after carbonization the situation turns opposite; addition of K+; and Na+ to anthracite or coal char can decrease ignition temperature and K+; is more effective than Na+;.

Self-ignition and mechanisms of interaction of coal with oxygen at low temperatures. 3. Changes in the composition of coal heated in air at 60 °C

The gradual changes in weight, in elementary composition, in content of oxygen functional groups and in spin concentration have been established for samples of black coal, brown coal and lignite heated in air for a maximum of 1200 h. An intensive oxidative dehydrogenation was found to occur after more than 300 h oxidation. The process was preceded by a decrease in weight and in oxygen content; its start was found to coincide with a sharp increase in the alcoholic content and in coal ability to exchange hydrogen with calcium acetate, and its rate was found to diminish simultaneously with this ability. The acidity ascribed to phenols decreases gradually but it was found to increase at the end of the experiment. A decrease in ignition temperature was established for oxidized black coal, as well as a change in the composition of low-molecular-weight hydrocarbons evolved during the subsequent gradual heating in air. Some of the changes in the composition during coal oxidation may be explained by acid catalysis.

The ignition of uranium

Experimental studies are reported of the ignition of pure uranium. Most of the experiments were performed by placing the metal samples in a flowing oxidizing atmosphere (either air or oxygen) within a furnace whose temperature was increasing at the rate of 10° C/min. The ignition temperature of the sample was determined graphically from the sample temperature-time record as the point of intersection between extensions of the pre-ignition heating rate and the post-ignition self-heating rate. The ignition temperature of 8.5-mm cubes of uranium from three sources (in air and in oxygen) was very close to 600° C; however, characteristic differences in reaction were apparent in the 400–500° C range. Metal in an “as-cast” condition self-heated to a greater extent in the 400–500° C region than did beta-quenched metal. Nitrogen-oxygen mixtures also increased selfheating in this region to a greater extent than did helium-oxygen mixtures. Ignition in air or oxygen was followed by spontaneous thermocycling. Peak burning temperatures in air were limited to about 1500° C, but in pure oxygen temperatures greater than 2150° C were recorded. Ignition temperature decreased with sample size. Studies with a series of cubes, wires and foils, with specific areas from 0.3 to 120 cm 2/g, yielded an irregular decrease of ignition temperature from 640 to 315° C in oxygen. The irregularity occurred with ignition in the temperature region 400 to 500° C and was shown to be associated with the transition of the oxide from an autocatalytic form to a protective form. A simple mathematical model based upon isothermal reaction rate laws and simple heat transfer laws yielded an ignition temperature-specific area relationship which agreed well with the experimental results and which predicted the irregularity. Experiments with aggregate samples, i.e., bundles of wires and stacks of foils, showed that ignition temperature decreased with increasing aggregation as expected from simple heat transfer considerations. It was concluded that uranium ignitions in air and oxygen are thermal ignitions which can be explained in terms of a balance of heat losses and heat generated by the oxidation reaction.