Burning Rate Of Particles Research Articles

Simultaneous particle morphology and temperature measurements of burning pulverized coal

Simultaneous, in situ measurements of the morphology, temperature and velocity of individual pulverized coal particles are presented. Particle temperatures are measured using two-color pyrometry and shapes are obtained from two-diemnsional images. From these images, estimates of particle external surfac area and volume are obtained using a discrete revolution technique. This method provides more accurate determination of the surface area of irregularly shaped particles than the equivalent sphere method typically referred to in the literature. The results of simultaneous morphology, temperature and velocity measurements obtained for a Pittsubrgh seam coal (35% volatile matter by weight) 30 msec after particle ignition in various bulk gas oxygen concentrations are presented. At oxygen concentrations of 10 and 15% by volume, two types of particles are observed: low temperature particles with large, irregular shapes and high temperature particles with more compact shapes. Examination of collected particles using scanning electron microscopy showed the low temperature particles to be lacy, soot-like formations and the high temperature particles to be partially burned coal particles. A comparison is made between calculated particle burning rates (μg/s) obtained using the discrete revolution technique and those obtained using the equivalent sphere method. Burning rates obtained from the images are approximately 30% higher than those obtained from the equivalent sphere method. The two-dimensional images provide a better estimate of the higher surfaces areas associated with irregular shaped particles and hence the higher calculated particle burning rates (μg/s) are more accurate.

Variations in the temperatures of coal-char particles during combustion: A consequence of particle-to-particle variations in ASH-content

Experimental evidence is provided that supports our hypothesis that the wide variations in the temperatures and burning rates of pulverized-coal char particles of nominally the same size are primarily a consequence of particle-to-particle variations in ash-content. In situ size, temperature, and velocity measurements on the char particles of two uncleaned, run-of-mine coals and on the char particles of their physically-cleaned products are presented. The cleaning process reduced the total mineral matter content of the coals. It is shown that the variations in particle temperatures are reduced with cleaning. The higher the mineral matter content of the coal, the wider the variations in the ash-content of individual particles and hence, the wider the variations in the temperatures of particles of nominally the same size. Wide variations in particle temperatures lead to wide variations in particle burning rates. It is also shown that the temperature-dependence of the chemical reactivity of the particle material remains unchanged throughout the early to late stages of burning. Further, the data indicate that catalysis by mineral matter is negligible at particle temperatures exceeding 1500 K.

The influence of transport phenomena on the fluidized bed combustion of a single carbon particle

The burning rate and temperature of the carbon particles are known to affect the efficiency of a fluidized bed combustor, and also the emission levels of undesired noxious components. The main results of an extensive study on the fluidized bed combustion behaviour of a single carbon particle [1] are summarized. Calculations have been carried out with a newly developed transient model, the ASPC model, and also with the much simpler progressive conversion model. Besides, many experiments have been performed in a lab-scale fluid bed construction to measure the burning rate and temperature of individual carbon particles for various conditions. From the comparison between experimental results and model predictions it has been overall concluded that the ASPC model is especially useful in i) describing the complex behaviour of progressive carbon conversion for the regime of combustion controlled by carbon reactivity plus intraparticle oxygen diffusion, and ii) estimating the conditions for which transition to the regime of external mass and heat transfer control occurs. Accurate prediction of the carbon particle burning rate and temperature is only possible for the latter combustion regime.

Reacting solid particles in one-dimensional nozzle flow

A fully conservative explicit scheme for a solid-particle-laden gas flow in a convergent-divergent nozzle is studied numerically as a direct time-dependent problem for a one-dimensional flow. The initial conditions at t = 0 are taken from the similarity solution. The boundary conditions at the nozzle inlet are determined by taking an arbitrary large cross-section at the first grid point before the nozzle inlet and are held constant with time. The exit boundary conditions are obtained at the grid point next to the exit plane from the values extrapolated at the exit plane. The particle burning rate is assumed to be a function of pressure. A time-dependent predictor-corrector formulation with a fourth-order damping term is used. Convergence of the result is assumed when the throat Mach number change is <0.0001, generally in ⩽ 400 time steps.

On the products of the heterogeneous oxidation reaction at the surfaces of burning coal char particles

The combustion of the char particles of two bituminous coals were investigated in a laminar flow reactor having gaseous environments containing 6 and 12 mole-% oxygen in the temperature range 1550 to 1700 K. Measurements of the size, temperature and velocity of individual particles were made at selected heights above the reactor inlet and hence, at different extents of burnoff. Samples of partially reacted chars, extracted from the reactor at these same heights, were analyzed in order to determine the fraction of the initial mass remaining, m/mo, and the char density. The data were used in the governing mass, momentum and energy balance equations in order to determine overall particle burning rates per unit external surface area. The burning rates determined indicate that account must be made of CO2 formation in the vicinity of the particle. Whether the CO2 is formed heterogeneously at the particle surface, or homogeneously in the particle's boundary layer cannot be established definitively. It is shown, however, that the single-film model of a burning carbon sphere with a fraction ψ of the carbon being converted to CO2 at the particle, surface adequately describes the burning behavior of coal char particles in the 75 to 125 μm size-range. Employing such a model, for the char particles of the bituminous coals investigated, as much as 15% of the carbon content of the particle can be converted to CO2 at temperatures in the range 1600 to 1700 K. At temperatures above 1800 K. CO is essentially the sole heterogeneous reaction product.

Promotion of char oxidation by inorganic constituents

A North Dakota lignite has been demineralized and selectively reloaded with calcium, potassium, and sodium cations by an ion-exchange process. Chars produced from the treated samples were burned in a laminar-flow reactor and single-particle temperatures were determined by optical pyrometry. Results show that sodium and potassium cations present in the chars have little effect on the char particle temperatures or burning rates. Calcium cations enhance the char particle temperatures at low concentration (<5000 ppm). The results are consistent with carbon dioxide being produced at the char surface by catalytic action of the char mineral matter.

Experimentally determined overall burning rates of pulverized-coal chars in specified O 2 and CO 2 environments

Measurements of the size, temperature and velocity of individual pulverized-coal char particles are used to determine the overall burning rates of particles flowing in gaseous environments of specified O2 and CO2 contents. The overall burning rates of the char particles are essentially unaffected by the carbon dioxide content of the ambient gas. When the single film model of a burning carbon particle with CO as the sole heterogeneous reaction product is employed, the chemical reaction rate coefficients determined in gaseous environments of fixed CO2 levels but varying O2 levels exhibit the same temperature dependence. For the Missouri bituminous coal studied, a reaction rate coefficient of 28.4 exp (−2000/RT) g/cm2-s is determined for the C−O2 heterogeneous reaction. The apparent activation energy is in the range expected for burning limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the particle material. Based on the overall particle burning rates determined, the Thiele modulus estimated for each particle size is in the range which suggests little penetration of oxygen into the particle pores. The results support the single film model of a burning particle with CO as the sole reaction product for coal chars burning under typical pulverized-coal combustion conditions. The results indicate that the double film model of a burning carbon particle is not applicable for particles less than about 130 μm burning under such conditions. A reaction rate constant for the C−CO2 heterogeneous reaction a thousand times greater than the rate constants reported in the literature is required in order to get agreement between measured and calculated temperatures for a given particle size when the double film model is employed.

Group combustion of char/carbon particles

Extensive literature exists for the experimental data on coal/char ignition and combustion. While most of the experiments are performed with a cloud or stream of particles, the theoretical modeling used to compare and interpret the experimental data is based on the individual particle combustion (IPC) model. As opposed to individual particle modeling, a group combustion (GC) theory is proposed for the combustion modeling of char/carbon particles. For a cloud of liquid drops, the group behavior implies the formation of a flame (group flame) around a large number of drops rather than a flame around each drop. More generally, the group behavior for a cloud of particles represents the change in the burning characteristics due to collective behavior of particles with or without a group flame. To gain a basic understanding of the group behavior, a model such as the analysis of a spherically symmetric cloud of particles burning in quiescent air is presented here. Each particle within the cloud produces CO, due to both the oxidation of C to CO and the reduction of CO 2 to CO which subsequently oxidizes to CO 2 in the homogeneous gas phase. Generalized results for the burning rate and the flame structure are given as a function of group combustion number (G). Predicted results show unexpected results including the independence of the burning rate of CO kinetics. Quantitative results for both the cases of frozen and fast CO kinetics are given. There is a group flame for the case of fast CO kinetics. It is shown that the group flame occurs at G > 5 while for a cloud of liquid drops, the group flame occurs at G > 0.1. The higher critical group combustion number is attributed to the lower burning rate of particle inside the cloud compared to the burning rate of liquid drops inside the cloud. The results show that there exists mainly three modes of combustion: (i) Individual Particle Combustion (IPC, low G), (ii) Group Combustion (GC, intermediate G) and (iii) Sheath Combustion (SC, high G). Criteria are given for identifying the mode of combustion from the experimental conditions. The criteria and the establishment of modes of combustion are independent of the extent of CO kinetics. It is found that the experimental data, obtained with a stream of particles and commonly interpreted with the IPC model, indicate the combustion modes to vary from IPC to SC modes. These data are now reinterpreted with the group theory.

Effect of multiple particle interactions on burning droplets

The interactions between burning droplets are analyzed on the basis of the quasi-steady assumption. A generalized treatment for burning rates of droplets in an array has been developed using a modified Laplace equation with point sources. This treatment is applied to two droplets of different sizes, as well as finite arrays containing up to eight symmetrically arranged monodisperse droplets. Particle interactions are shown to be a function of particle size, number density, and geometry of the array. Results are presented in terms of correction factors from which multiple particle burning rates can be calculated from single particle burning rates. The correction factors are shown to be in close agreement with results available in the literature.

The Motion of Particles Inside a Droplet

Equations of motion for particles translating inside a droplet with internal circulation were developed. The forces experienced by these particles and their relative importance are also summarized. Sample calculations for coal particles in oil droplets show that for high-droplet Reynolds numbers, particles will accumulate near the droplet surface before substantial evaporation of the droplet, based on the results that the characteristic time of transport is two orders of magnitude smaller than the droplet lifetime. These accumulations of particles near the droplet surface will probably result in particle agglomeration. Fow low-droplet Reynolds numbers, the particles are making slow looping motions and the characteristic time of transport is comparable to the droplet lifetime. This results in a more even distribution of particles in the droplet with slightly higher particle density near the surface. This indicates a higher particle burning rate and the possibility of internal boiling and microexplosion due to heterogeneous nucleation initiated from those coal particles.

On the temperature and reaction rate of burning pulverized fuels

A two-color pyrometer technique has been used to measure the temperature of burningpulverized fuel particles flowing in dilute suspension on the centerline of a laminar flow reactor. The transparent reactor is fed by a premixed flat flame fueled with CH 4 −H 2 −O 2 −N 2 mixtures, which permit widely variable post-flame temperatures and oxygen concentrations to be obtained. The two-color system employs an optical-electronic system which includes two monochromators to acquire light emission intensity at two wavelengths simultaneously from individual burning particles passing through a 1 mm square area on the centerline of the reactor. Planck's law and the gray body assumption (verified here by measurements) are used to compute particle temperatures, and the average temperature is then determined by averaging the values for some 500 individual particles. Particle temperatures have been determined for various gas temperatures over a range ofoxygen mole fractions from 0 to 0.2 for size-graded (90 μm mean diameter) samples of a petroleum coke and a flash pyrolysis char. Particle burning rates per unit external surface area were determined from the measured temperatures by a heat balance analysis. The functional variation in burning rate with temperature was then used to infer whether the burning rate was controlled by chemical rates (Zone I), the combined effects of chemistry and pore diffusion (Zone II), or bulk diffusion (Zone III). For the petroleum coke, both zone I and Zone II conditions were observed, and chemical rate coefficients for each zone were derived. For the char, Zone II and a transition to Zone III conditions were observed, and a chemical rate coefficient for Zone II was derived. Reaction orders and activation energies were in the range expected from previous oxidation studies, and the chemical rates for both samples were in good agreement with previous measurements by a different method in another laboratory. The results show that careful temperature measurements can provide a sensitive method for the determination of pulverized fuel burning rates.

Predictions of Intrinsic Ash Emissions From Wood-Waste Boilers

A primary cause of particulate emissions from wood-waste fired boilers is the suspension burning of solid fuel particles. These small particles are entrained by the upward flow of furnace gas and partially burn during their passage through the furnace. Because of the high moisture content of the wood fuel the furnace temperatures are low and, consequently, particle burning rate is low. As a result, the carryover of particulate matter consists of both combustible char particles and noncombustible intrinsic ash and tramp solids which are released as the wood material is consumed. The physical processes responsible for particle combustion and entrainment have been previously studied with a computer model specifically developed for wood-waste fuel. In the present work this model is used, along with three fuel size distributions, to predict the mass loading of intrinsic ash at the stack of a spreader stoker boiler as a function of operating conditions. The results indicate that the amount of intrinsic ash in the stack flow increases with increased combustion rate of the particles, which confirms physical intuition. The particle combustion rate increases with either increased fuel firing rate or decreased fuel moisture and excess air. These emission results for intrinsic ash, along with previous results for combustible carryover, are compared with recent data obtained from several operating wood-waste fired boilers.

Convective combustion modeling applied to deflagration-to-detonation transition of HMX

A two-phase reactive flow analysis has been applied to predict the possibility of deflagration-to-detonation transition (DDT) in packed beds of granulated HMX. The unsteady fluid mechanics during the ignition and flame-spreading sequence through porous beds provides a rapid pressure buildup, suggesting the formation of a shock wave. A specific criterion of critical P 2 τ was interpreted as satisfying requirements that would lead to the onset of detonation. A parametric study of granulated HMX has shown that important variations of the runup length to detonation exist for parameters that relate directly to the explosive mass and energy generation rates. These sensitive parameters are particle diameter, porosity, burn rate, and chemical energy of the explosive. Smaller effects due to drag, heat transfer correlations, ignition criterion, and numerical smoothing techniques do not seriously affect runup length magnitudes and are essentially uncoupled from the effects of the sensitive explosive parameters in establishing trends.

Combustion of carbon particles generated from liquid hydrocarbon fuels

The burning rate of residual carbon particles generated from the spray combustion flame of heavy fuel oil droplets has been investigated experimentally. The rate of combustion of a single particle and of a cloud of particles, and the processes of their combustion, have been analysed by electronic computer; the actual particle temperature has been taken into account. The theoretical results agree with the experimental results in their trend (Figs. 7, 10).

Combustion of Pulse-Heated Single Particles of Aluminum and Beryllium

Abstract This paper describes experimental studies of the combustion of single particles of aluminum and beryllium in a variety of oxidizers. Experiments were performed using both a pulsed Nd-glass laser and a xenon flash heating device as ignition sources. A scanning electron microscope (SEM) served as the major diagnostic tool in particle quenching studies. Major differences in the combustion of aluminum and beryllium have been found and have been related to the thermophysical properties of the metals and their respective oxides. New data on the burning rate of laser-ignited aluminum particles are presented. Scanning electron microscope pictures of the evolution of geometry of burning beryllium particles are presented. The effect of water vapor on the combustion of beryllium particles is documented. A discussion of the mechanism of metal particle fragmentation is included.