Particle Burn Times Research Articles

95/00652 A study on the combustion characteristics of PVC, poly(styrene), Poly(ethylene), and Poly(propylene) particles under high heating rates

Experiments were conducted in an externally heated drop-tube furnace to assess the effectiveness of the chemical calcium magnesium acetate [CMA, CaMg2(CH3COO)6] as a combustion catalyst and a coal pretreatment agent for reducing SO2 emissions. Bituminous coal particles of two distinct sizes, pulverized (75–90 μm) or micronized and beneficiated (mean diameter ∼3.5 μm) were burned. To measure the coal particle temperatures and burn times, combustion traces were recorded for single pulverized coal particles and clusters of micronized coal particles using a three-colour near-infrared optical pyrometer. The volatile and the char phase combustion temperatures of untreated pulverized-grind particles, in air at a gas temperature of 1450 K, were determined to be 2200 and 1800 K, respectively. Particles treated with CMA under the same conditions burned hotter, with the temperature of volatile and char phase being 2400 and 2000 K, respectively. SO2 and NOx concentrations were measured at the exit of the furnace for both the pulverized and the micronized coals. For furnace gas temperatures between 1250 and 1450 K, in a background gas containing 10–50 ppm SO2 and equivalence ratios, π, between 0.4 to 0.7, untreated micronized and pulverized bituminous coal particles produced SO2 emissions in the range 100–200 ppm and NOx emissions in the range 200–450 ppm. In contrast, combustion of pulverized particles treated with CMA, burning under the same conditions, not only did not produce any SO2 but also eliminated the background SO2 concentration. The combustion of micronized coal treated with CMA produced less SO2 than untreated micronized coal, but complete reduction of SO2 was not achieved. Experiments with CMA-treated micronized coal in atmospheres containing 40% oxygen suggested that the primary mechanism for sulfur removal in this virtually ash-free coal was the sulfation of resulting CaOMgO fly ash/aerosols. Experiments in which the effluent of the combustion of pulverized coal was quenched immediately after the heated furnace zone suggest that a fraction of the fuel sulfur (up to 50% of the released SO2) may have been encapsulated by ash during the combustion of treated pulverized coals, with the remaining released SO2 being removed by sulfation of CaMg-containing ash or submicron CaOMgO aerosols in the cool-down region of the furnace.

A Simplified Model for the Combustion of Uncoated Boron Particles

Abstract The combustion of an uncoated boron particle in an oxidizing environment is theoretically studied. Chemical reactions considered include the surface B-O2 and B-B2O3 reactions and the gas-phase BO-O2 reaction. Generalized coupling functions are identified and three limiting solutions respectively describing frozen, detached flame-sheet. and attached flame-sheet combustion for the gas-phase reaction are derived. Effects of reactivities, surface temperature, pressure. particle size, and oxide condensation are studied, while the importance of finite rale surface reactions are also demonstrated. Approximate expressions are derived for the particle burning time; the predictions compare well with the experimental data of Macek.

Burning Rates of Lignite Particles at Low Furnace Temperatures

Abstract Experimental burning times of particles were obtained for a sample of a Type A lignite, screened between 88 and 105 im, in a laboratory furnace with essentially cold walls at a constant gas temperature of U50±20 K. The mass fraction of oxygen in the furnace, X,, ranged from about 0.20, below which no combustion was observed, to 0.46. Over most of this range, the combustion of particles was essentially complete. The measured burning times were short (30 to 50 msec) and are shown to be inversely proportional to the gas phase mass transfer rates, suggesting diffusion-controlled kinetics. There is only a narrow range, 0.20 < Xg < 0.23, in which the burning times were clearly higher, indicating an intermediate regime. It was also observed that, in this narrow range, the particles tended to extinguish before complete combustion. Thus, it appears that, as X, was progressively decreased, there was unusually rapid transition from the diffusion-controlled regime to extinction at relatively high values of X...

Effect of oscillating flow on combustion rate of coal particles

The burning times of captive coal particles (1–3 mm in diameter) in oscillating flow of controlled velocity amplitude and frequency have been measured. This study was undertaken to explain the high values of combustion intensities reported in experiments with pulsating combustors. The experimental data, presented herein, indicate that higher values of the ratio of the air displacement amplitude to the initial radius of the coal particle result in greater reduction in the burning time. The experiments also revealed the existence of two-stage combustion for the residue: shell burning, for which the combustion rate is controlled by the rate of oxygen transport in the gas phase; and core burning, for which the combustion rate is controlled by the rate of diffusion of oxygen through the porous ash layer formed during combustion. A theoretical model is discussed which explains the effect of the ash layer on the combustion rate of the particle. This model shows that the reduction in burning time of a particle of ash-forming coal (for which the ash does not flake off) caused by the enhanced rate of oxygen transport in the gas phase may be significantly less than the corresponding reduction in burning time of a coal particle for which the ash layer is absent.

A model for coal dust duct explosions

A theoretical discussion is given of the propagation of a dust explosion in a linear duct or pipeline. The particular aim is to investigate the experimental observation that propagating explosions are much harder to initiate in small laboratory scale ducts than in, say, coal mine galleries. A model is proposed in which a turbulent mixing phenomenon first identified by G. I. Taylor gives, for large ducts, very high flame velocities, which in turn lead to large fluid velocities and further increases in flame velocity. In small ducts, the time scale of the turbulent mixing is less than the time needed for the burning of individual coal particles. The particle burning time becomes an additional constraint on the rate of flame propagation and the development of explosions is inhibited.

Detection of boundary-layer transition with a laser beam

Fig. 5 burning time of particles as a function of the oven temperature. References 'Schadow, K., 'Study of Gaseous Nonequilibrium Effects in Particle-Laden, Ducted Flows for Improvement of the Combustion Efficiency, AIAA Paper 72-36 San Diego, Calif., 1972; Schadow, K., The Influence of Combustor Parameters on the Combustion of Particle-Laden Fuels in Ducted Flows, AIAA Paper 73-177, Washington, D.C., 1973. Boussios, A., Primarkammerprozesse fur Kombinationsantriebe, Kursus der Carl-Cranz-Gesellschaft uber Flugkorperantriebe in Braunschweig, March 19-23,197?. McLain, W.H., Identification of Exhaust Species from the combustion of LM and LMH Fuels, AFRPL TR-68-105, April 1968, Air Force Rocket Propulsion Lab., Edwards Air Force Base, Calif. Markowskii, L.Ya. and Kondrashev, Yu.D., The composition and Properties of Borides of the Elements of the first and second Groups of the periodic Systems, Zhurnal Neoranicheskoi Khimii, Vol. 11, No. 1, 1957, pp. 34-41. Secrist, Childs, US Atomic Energy Rept. TID-17149, 1962. Hsia, H., Air-Augmented Combustion of Boron and BoronMetal Alloys, Final Rept. AFR DL-TR-71-80, June 1971, Air Force Research Lab., Wright-Patterson Air Force Base, Ohio.

Combustion of boron particles: Experiment and theory

Burning times of single boron particles were measured as functions of three experimental variables: particle diameter (d) from 37 to 124 μm, pressure (P) from 0.17 to 35.0 atm, and mole fraction of oxygen in the gas (X) from 0.1 to 1.0. Limited studies were also made in fluorine-containing gases without oxygen. A previously described laser-ignition technique was used. Measurable pre-ignition delays were observed in oxygen-containing gases, but not in the absence of oxygen. The effect of convection in these free-fall experiments was found to be significant at large values of d and P, and had to be taken into account in interpretation of the data. When d, P, and X are sufficiently large, the burning times are found to be inversely proportional to X, directly proportional to d2, and independent of P. At low values of d, P, and X, burning times deviate from these scaling laws. The flammability limit is between X=0.10 and 0.21. Quantitative agreement with a gas-phase diffusion theory is good under conditions well removed from the flammability limit. Burning rates with fluorine as oxidant are lower than rates with oxygen, consistent with diffusion considerations.

Combustion and ignition of particles of finely dispersed aluminum

1. It has been established that the burning time of individual aluminum particles in a high-temperature gas flow is almost independent of the pressure (P>20 atm) and temperature (T>2000° K) of the medium, but is a very strong function of the oxidizing properties of the flow relative to the concentration of active oxygen-containing products (H2O, CO2, etc). The formula τb0==0.67(d1.5/(akr)0.9) is suggested for calculation of the particle burning time. This gives satisfactory agreement with experiment. 2. The ignition time for aluminum particles is insensitive to the composition of the oxidizing medium and pressure, but depends strongly on the temperature of the gas flow. The increase in ignition time as the particle diameter increases is proportional to d2.

The Combustion of Premixed Laminar Graphite Dust Flames at Atmospheric Pressure

Abstract A convenient apparatus for dispersing uniform clouds of solid particles for laboratory dust flame experiments has been tested. The combustion characteristics of premixed laminar graphite-oxygen-nitrogen flames have been studied at atmospheric pressure both with and without added hydrocarbon. The variation of particle burning time with initial oxygen partial pressure and oxygen/ fuel ratio has been described by first-order combustion kinetics with due consideration for oxygen depletion during burn-out. Flame velocity measurements indicate very slow propagation compared with hydrocarbon sprays and aluminum clouds of similar particle diameter. Added chlorine did not measurably affect graphite combustion up to concentrations of 2.5 vol. % in the presence of added hydrocarbon, but less than 1.0 vol. % of chlorine extinguished pure graphite-oxygen flames. The addition of 1.5 to 4.75 wt.% of lead acetate visibly stabilized the graphite-oxygen flame, but the effect on the flame velocity and burning rate ...

Ignition and combustion of aluminum and beryllium.

This paper presents a summary of the results of theoretical and experimental studies on metal ignition and combustion conducted at the United Aircraft Corporation Research Laboratories. The burning times of spherical metal particles in rocket motors have been predicted using a computer program based on quasi-steady-state vapor-phase combustion. This program has been applied to the burning of aluminum or beryllium in oxygen, water vapor, or mixtures of one or both of these oxidizers with inert gases. The minimum conditions for ignition and the ignition delay time have also been calculated for metal particles in rocket motors as functions of gas temperature and particle size. Experimental measurements of the ignition temperatures of aluminum and beryllium have been made using electrically heated wires in static oxidizing atmospheres. The results obtained indicate that the ignition temperatures are dependent on the metal species, the pressure, and the oxidizer concentration and species. Of particular importance is the surface treatment or condition of the metal. The ignition temperatures of aluminum and beryllium can be significantly below the melting points of their respective oxides. 2239

The Influence of Coal Rank on the Burning Times of Single Captive Particles

Measurements of volatile and residue burning times of captive coal particles have shown them to vary in proportion to the square of the initial particle diameter; this was true for each of 10 coals tested. For the volatiles, the measured constant of proportionality was found to be independent of coal rank when allowance was made for swelling, the burning rate apparently being controlled by the diffusional rate of volatile escape through a porous matrix of fixed carbon. For the residues, the experimental “square-law” relation confirms Nusselt’s prediction; and agreement between predicted and experimental values of the proportionality constant was found to be good. An apparent rank dependence, showing shorter burning times with less fixed carbon, was simply a consequence of there being less fixed carbon to burn. There was no influence of rank-dependent “reactivity.” The relevance of the results to flames is also discussed.

Closure to “Discussion of ‘The Influence of Coal Rank on the Burning Times of Single Captive Particles’” (1963, ASME J. Eng. Power, 85, p. 189)

Closure to “Discussion of ‘The Influence of Coal Rank on the Burning Times of Single Captive Particles’” (1963, ASME J. Eng. Power, 85, p. 189)

Oscillatory burning of solid composite propellants

Analytical consideration of the interaction of an oscillatory pressure disturbance with the burning zone of a composite propellant leads to the conclusion that acoustic waves induce periodic variations in the ratio of the mass release rates of composite propellant components that give rise to harmonic composition and adiabatic flame temperature fluctuations in the gases above the propellant. These mass-release-rate, composition, and flame-temperature fluctuations may make large contributions to the amplification of acoustic waves by burning composite propellants. The magnitudes of these contributions to the admittance are particle-size-dependent and decrease as the ratio of particle burning time to oscillation period decreases. In contrast to homogeneous propellants, the amplification tendency of composite propellants is not expected to be a strong function of the burning rate pressure index of the over-all propellant. On the other hand, composition and flame-temperature contributions to the admittance may obtain for double-base propellants at high frequencies. Experimental work shows that bands of varying luminosity are released from the burning surfaces of propellants that are burned under the influence of an oscillatory pressure. The release of these bands can be related to composition and temperature fluctuations in the gases, but as yet not unambiguously.

Radiative transfer in dust flames

This work presents an overview about the explosion behaviour of metallic powders from micron to nanosize. Aluminium, magnesium, titanium, iron and zinc were considered and their explosion safety parameters were analysed as a function of their mean primary particle size either determined by BET measurements, particle size distribution. To depict the course of explosion behaviour for these metals, extensive literature review has been performed and additional experimental tests were also performed. Generally, decreasing the particle size in a metallic powder leads to a higher explosion severity. It appears that this statement is true till a critical diameter below which the explosion severity (pmax, dp/dtmax) decreases for all the considered powders. This critical size can be explained by theoretical considerations on the nature of thermal transfer in the flame, namely by analysing the Cassel model. Finally, semi-empirical models were also developed for aluminium to highlight the specific micrometre and nanometre behaviour and the influence of turbulence, particle burning time, diameter and concentration. The influence of these key parameters needs to be further assessed in a future work in order to better understand the mechanisms involved and to extend the scope to other powdered materials.