Total Burning Time Research Articles

Determination of overall burning rate coefficient of coal-oil droplet mixtures

Theoretical considerations of the combustion of coal-oil droplets were studied, and formulae for the total combustion time of the single droplet and for the overall burning rate coefficient were derived. To verify the formulae, experimental investigations were carried out using forced ignition. The theoretical model gave a good approximation to the experimental data only for the boiler coal. The overall burning rate coefficient was about one quarter that of heavy oil, with 50% gram fracture of coal. For lean coal, the overall burning rate coefficient was of the order of the heavy oil burning rate coefficient.

An experimental investigation of flame behavior during explosions in cylindrical enclosures with obstacles

Flame behavior during explosions in cylindrical enclosures with obstacles has been investigated. Photographic records show the influence of a wire grid, a circular plate, and an orifice on the flame's shape. The reduction of total combustion time is measured by pressure recordings. It is shown that the acceleration of the combustion process is greatest with the orifice. The instantaneous flow velocity was measured behind the orifice during the short explosion process using a laser-Doppler anemometer. Typical characteristics of the velocity distribution were found to be similar to those obtained in steady flows.

Experimental Study on Evaporation and Combustion of Single Coal-Slurry Droplets (Behavior on a Hot Surface in an Elevated-Temperature Environment)

The combustion characteristics of single CWM and COM droplets on a hot surface were investigated. The surrounding temperature was carefully adjusted to the temperature of the hot surface. The vaporization and combustion behavior of a single slurry droplet under a uniform temperature condition could then be observed. The ignition delay time of the COM droplet was shorter than that of the CWM droplets. When the initial diameter of the droplet decreased, the total burning time decreased proportionately with the square of the initial diameter. The effect of the size distribution of the pulverized coal appeared on the lower temperature limit of the surface reaction process of the fixed carbon. Ten kinds of pulverized coal were compared to obtain the relation between the fuel ratio of the coal and the apparent diffusion coefficient that controlled the combustion of the slurry droplet.

Experimental study on evaporation and combustion of single coal slurry droplet. 1st report. Behavior on a hot surface in a elevated temperature environment.

The combustion characteristics of a single CWM and COM droplet on a hot surface were investigated in a test furnace in which the ambient and surface temperature were equivalent ; this is to say, were at a uniform temperature. The droplet was simply set on the hot surface without using any suspending equipment like a silica thread. The following results were obtained. The ignition delay time of the COM droplet was shorter than that of the CWM droplet. The total burning time of the COM droplet was only slightly shorter than that of CWM. When the initial diameter of a droplet decreased, the total burning time was proportionally decreased by the square of the initial diameter. The effect of size distribution of the pulverized coal appeared on a surface combustion process of fixed carbon. The effect of coal properties on the surface combustion process was also studied using ten kinds of coals.

Mechanism of wood combustion: 2. Weight loss analysis

The combustion of moisture-free wood cylinders was studied by measuring weight loss at 750 °C. The rate of weight loss increased for an initial period of ≈ 60 s; thereafter it decreased asymptotically in an overall first-order reaction. Particles with protected ends showed a characteristic total burning time that was exponentially proportional to the initial diameter of the particle.

Flame initiation in a spark-ignition engine

These results indicate that a laminar like burning process immediately follows the spark discharge. This was found to last over a significant part of the total combustion duration (about 15 degrees compared with a total burn time of about 80 degrees), but only a negligible fraction (10 −3) of the mass is burned. The convection velocity of the kernel center during that period indicates fluid motion in the region of the spark plug in the range 2–4 m/s. Since the lens used in the Schlieren setup produced some distortion to the flame sihouette when the kernel moved significantly off the lens axis, flame growth rates beyond 15 degrees after the spark were not examined. Nevertheless, this study clearly indicates a laminar growth of the flame in its early stages of development.

Burning Rate of Millimeter Sized Wood Particles in a Furnace

Abstract Burning rate data for single, mm sized, pine and oak cubes in a convective flow furnace are presented. The gas temperature was varied from 900 to 1200 K, the oxygen concentration from 0 to 21 percent, the particle Reynolds number from 60 to 240, the moisture content from 0 to 200 percent, and the size from 5 to 20 mm. The devolatilization stage accounts for 80-90 percent of the mass loss and 50 percent of the total burn time. The devolatilization rate of a given particle is influenced by a furnace temperature, Reynolds number and oxygen concentration. The implications of the experiments for modeling of wood particle combustion are discussed.

Conceptual Design of a Moving-Ring Reactor

A design of a prototype moving-ring reactor was completed, and a development plan for a pilot reactor is outlined. The fusion fuel is confined in current-carrying rings of magnetically field-reversed plasma (''compact toroids''). The plasma rings, formed by a coaxial plasma gun, undergo adiabatic magnetic compression to ignition temperature while they are being injected into the reactor's burner section. The cylindrical burner chamber is divided into three ''burn stations.'' Separator coils and a slight axial guide field gradient are used to shuttle the ignited toroids rapidly from one burn station to the next, pausing for one-third of the total burn time at each station. Deuterium-tritium-/sup 3/He ice pellets refuel the rings at a rate that maintains constant radiated power. The fusion power per ring is approx. =105.5 MW. The burn time to reach a fusion energy gain of Q = 30 is 5.9 s.

石炭スラリーの燃焼

Combustion of coal slurry fuels are discussed and reviewed. Basically, the combustion life time of a coal slurry droplet is under the control of residual agglomerates combustion period, and dispersion medias are classified into adhesive and non adhesive media on the basis of which they generate residual char or coke as the adhesion paste or not.The burnout time of coal slurry fuels is discussed by using the single droplet combustion behaviour of each slurries, also burning characteristics are discussed through fundamental research results.From the point of view of total burning time, the atomisation is an important technology for improving the combustion. Recent works on the slurry atomisation is reviewed. Also, problems on the coal slurry combustion within a practical furnace are described.

Single-droplet combustion of coal slurry fuels

The combustion characteristics of single droplets of coal slurry fuels were experimentally investigated using both spontaneous and forced ignition. Results showed that the combustion is a sequential two-staged process, consisting of gas-phase combusion of the volatiles followed by combusion of the solid residue, which is mostly carbon. A splashing combustion phenomenon, which corresponds to the outgassing of the thermally cracked fuel gas from the droplet interior, occurred during the later period of gas-phase combustion in the same way as for residual heavy fuel oils. However, no disintegration or disruption of the droplet was observed. The duration of combustion of the solid residue agglomerate was several times longer than that of the volatiles, and therefore constituted a major part of the total burning time. The gas-phase burning time as well as the total burning time were found to vary linearly with the square of the initial droplet diameter, thus obeying the “square law of initial droplet diameter.” The overall burning rate coefficients of coal slurry fuels were about 1 2 to 1 4 times those of residual heavy fuel oils because of the long solid combustion time. The overall burning rate coefficient was found to depend mainly on the residual carbon content of the coal slurry fuel.

Combustion and Agglomeration of Coal-Oil Mixtures in Furnace Environments

Abstract The combustion characteristics of freely-falling and suspended mono-sized coal-oil mixture (COM) droplets in heated air were investigated by studying the quenched samples using scanning electron microscope. Parameters varied include the volatility of the oil, the concentration and size of the coal particles, the residence time, the intensity of blowing, and the effects of water and surfactant addition. Results substantiate the formation of coat particle agglomerates upon depletion of the volatile oil components, and emphasize the importance of oil volatility and external convective motion on COM combustion. In particular agglomeration is not expected to be a serious problem for COM with No. 6 oil, although addition of oils with higher volatility widens the volatility differentials between the oil and coal components such that the agglomerate formation, heating, ignition, and thereby the total burning times are significantly prolonged. Addition of water may cause droplet swelling and fragmentation...

Combustion of single coal particles in a jet

Fluidized bed combustion has attracted much interest in recent years, but there is very little data on the behavior of coal particles at these new conditions. Coal of much larger diameter (1–10 mm), much lower furnace temperatures (~850 °C), and different fluid mechanical conditions exist compared to pulverized coal furnaces. This paper presents experimental data on the behavior and combustion rates of individual coal particles aerodynamically suspended in a heated jet, to stimulate flow conditions in a fluidized bed. Tests of bituminous, sub-bituminous and lignite coals from 2 to 12 mm at jet temperatures of 705 and 816 °C in air and air diluted with equal parts of nitrogen were conducted. The ignition delay time varied from 2 to 44 sec. The devolatilization time extended up to 80 sec and was dependent mainly on particle size. The total burn time was independent of coal type and temperature, and varied as the square of the size and inversally with the oxygen concentration. The total turn time varied from 25 to 740 sec independently of coal type. The square law for the char burning rate was investigated.

The problem of liquid droplet combustion—A reexamination

The simple quasi-steady analysis of the combustion of a liquid fuel droplet in an oxidising atmosphere provides unsatisfactory explanations for several experimental observations. It's prediction of values for the burning constant ( K), the flame-to-droplet diameter ratio ( d f d s ) and the flame temperature ( T f ) have been found to be amgibuous if not completely inaccurate. A critical survey of the literature has led us to a detailed examination of the effects of unsteadiness and variable properties. The work published to date indicates that the gas-phase unsteadiness is relatively short and therefore quite insignificant. A new theoretical analysis based on heat transfer within the droplet is presented here. It shows that the condensed-phase unsteadiness lasts for about 20–25% of the total burning time. It is concluded that the discrepancies between experimental observations and the predictions of the constant-property quasi-steady analysis cannot be attributed either to gas-phase or condensed-phase unsteadiness. An analytical model of quasi-steady droplet combustion with variable thermodynamic and transport properties and non-unity Lewis numbers will be examined. Further findings reveal a significant improvement in the prediction of combustion parameters, particularly of K, when consideration is given to variations of c p and λ with the temperature and concentrations of several species. T f is accurately predicted when the required conditions of incomplete combustion or low ( O F ) at the flame are met. Further refinement through realistic Lewis numbers predicts ( d f d s ) meaningfully.

Boron Carbide Dust Flames

Abstract The total burning time of boron carbide powder has been measured in premixed laminar boron carbide-propane-oxygen-nitrogen flames at atmospheric pressure. Burning times were several times longer than for elemental boron of the same particle size and were even longer than predicted by extrapolation from larger single-boron carbide particle burning experiments. Several possible explanations for this discrepancy are discussed.

Amorphous Boron Dust Flames

Abstract The total burning time of amorphous boron powder has been measured in premixed laminar boron-propane-oxygen-nitrogen flames at atmospheric pressure. The variation of burning time with initial oxygen partial pressure has been described by first-order combustion kinetics with due consideration for oxygen depletion during burn-out. Measured burning times are clonsistent with those reported previously for both single crystalline boron particles and amorphous boron agglomerates. Attempts to establish boron-oxygen flames indicate extremely slow flame propagation in the absence of added hydrocarbon.

Combustion of boron particles at elevated pressures

Combustion of single boron particles, about 75 microns in diameter, from a crystalline powder sample has been studied. Particles were, ignited by being dropped through a focused laser beam in several oxidizing gases over a range of pressures In pure oxygen, in air, and in O2/Ar (20/80), particles were merely preheated to a temperature about 2000°K; ignition took place spontaneously after a measurable induction period. Quantitative values of both the induction period and the subsequent self-sustained combustion period are listed. In air and in the O2/Ar mixture, the burning times decrease from about 45 msec to about 20 msec as the pressure is increased from 1 to 35 atm. In pure O2 at atmospheric pressure, the total burning time is only 6.8 msec. In pure CO2 and in O2/N2 (7/93), there is no induction period, no selfignition, and no steady-state combustion; particles must be brought to a burning regime by an external energy flux, and they are able to maintain that regime for only a limited time before extinguishment. It is shown that the classical theory of ignition and combustion can account for all three observed burning modes: metastable surface reaction during the pre-ignition period rapid self-sustained diffusion combustion, and decaying combustion. Previously reported reaction-rate and ignition-limit data are used for quantitative estimates of parameters pertaining to the three regimes.