Small Coal Particles Research Articles

Rate Measurements in a Laboratory-Scale Pulverized Coal Combustor

Abstract An atmospheric combustor has been developed to investigate the details of pulverized coal combustion. Gas and char samples were obtained from the combustor with water-quench probes. An argon tracer was used to determine the extent of gas mixing while the extent of particle dispersion and the extent of particle reaction were determined from measured char quantity and composition. Secondary air velocity and injection angle and initial coal size were varied in 28 tests. Secondary velocity had a significant impact on gas mixing rate while particle dispersion was not influenced significantly by changes in test variables. Rates of the several processes which influence pulverized coal combustion were determined. Gas mixing, coal particle heat-up, particle dispersion and coal particle reaction times were all of comparable magnitude and were all reduced by use of smaller coal particles. Measured profiles for mixture fraction, gaseous species, particle flux and coal burnout percentage provide data for comb...

Product compositions and formation kinetics in rapid pyrolysis of pulverized coal—Implications for combustion

Previous work in this laboratory on the rapid pyrolysis of a lignite and a bituminous coal has been extended to include time-resolved measurements of volatiles composition. Approximately monolayer samples of small coal particles supported in a metal screen were rapidly heated in helium to a desired temperature, held there for a desired time, and then rapidly cooled. Volatiles were rapidly diluted and quenched, and all products were collected. Sample weight loss and product compositions were determined as a function of heating rate (270-10,000°C/s), peak temperature (up to 1100°C), holding time at peak temperature (0–30s), pressure (0.0001–100 atm), and particle, size (74–1000 μm). Volatile yields increase montonically as temperature increases, with the evolution of different products or groups of products occurring in sequential but overlapping temperature intervals. The lignite volatiles are dominated by CO, CO2 and H2O, while the main volatiles from the bituminous coal are tar and light hydrocarbons. Heating rate has negligible effect on total yields or product distributions from either coal over the range studied. Pressure and particle size have little effect on product yields from the lignite. In contrast, for the bituminous coal, increases in pressure or particle size result in less tar and more char and hydrocarbon gases, apparently reflecting mass transport limitations and the occurrence of secondary reactions within or on the coal particles. The kinetics of the evolution of different products from the lignite is successfully modeled by one, two or three first-order decomposition reactions. For bituminous coal, a similar model which includes evaporation and diffusion of tar along with pyrolytic and secondary reactions is found to be consistent with the observed behavior. Use of the data to calculate the heating value and surface flux of volatiles under conditions pertinent to pulverized coal combustion indicates, in agreement with previous work, that combustion on the particle surface may in some cases precede the occurrence of a volatiles flame away from the surface.

Bubble nucleation in viscous material due to gas formation by a chemical reaction: Application to coal pyrolysis

AbstractMost coals swell when heated, and their plastic properties change. These phenomena are explained qualitatively and semiquantitatively using a theory which combines classical nucleation theory and chemical kinetics.Classical bubble nucleation theory is modified to include cases where gases are formed in a liquid as a result of a chemical reaction. Part of the gas that is formed in the liquid escapes through the surface. However, the concentration of gas that remains in the solution increases until a critical concentration is reached. When the critical concentration of gas in the solution is reached, bubbles will begin to nucleate.The value of the rate of nucleation has a critical upper limit which is determined by the thermodynamic properties of the solution. However, the kinetics of the heat, momentum, or mass transfer may reduce the thermodynamic rate.The criteria to decide which transport mode limits the kinetics of the nucleation were derived and applied to melting coal. In coal, the rate of bubble nucleation appears to be limited by the rate of momentum transfer in the melt.The size of the melting coal particle determines the ratio of surface area to volume and thus affects the kinetics of the accumulation of gaseous reaction product in the melt. Thus, smaller rates of bubble nucleation will be observed in smaller coal particles (very viscous melts).Kinetic equations are derived and used to estimate the time that is required for bubbles to appear inside a coal particle. A general method is proposed to calculate the time‐temperature contour at which nucleation will occur. The effect of the particle size on the time of nucleation is calculated. The results of the calculations yield values which compare very well with experimental results on coal.

A theory on transition of ignition phase of coal particles

A century-long held notion was that the ignition of coal particles always occurs in the gas phase, as in a totally pyrolysing material. Recent experimental results disprove such a sequence, at least for small bituminous coal particles. Coals undergo only partial pyrolysis and hence ignition could occur in one of two possible modes: (i) exothermic gasification due to heterogeneous oxidation (regime I), and (ii) endothermic pyrolysis and subsequent oxidation of the pyrolysate in the gas phase (regime II). A steady-state ignition theory is developed for partially pyrolysing solid particles. The theory is formulated on an asymptotic basis: regime I is assumed to proceed independent of regime II. For regime I, Semenov's thermal theory is used to obtain the heterogeneous ignition temperature (HIT) and the solutions reveal that an increase in particle size and oxygen concentration results in decrease of HIT. For regime II, an adiabatic criterion is invoked to define the gas ignition temperature (GIT). The solutions reveal that a decrease in particle size and an increase in oxygen concentration increases the GIT. Superimposing the results for HIT and GIT, the conditions for transition of ignition phase (TIP) are found. The Pittsburgh seam bituminous particles of size below 350 μm may not undergo gas phase ignition, confirming recent experimental results.

Laser Heating of Coal Particles in the Source of “Time-of-Flight” Mass Spectrometer

DURING general investigation of the processes that occur in the combustion of pulverized coal, experiments have been undertaken to develop methods for determining the nature of products formed by the rapid thermal decomposition of coal particles, especially those evolved in the very early stages of decomposition. In previous work1 the yields of the stable decomposition products evolved from small coal particles during the first millisecond of flash heating were determined by quenching the products in an inert atmosphere. Using this method, however, it was not possible to determine whether free radical fragments constituted part of the primarily evolved species. Recent work on the pyrolysis of lump coal using a laser beam has suggested that free radicals of molecular weight of about 100 are evolved2, but in these experiments the products arose both from complete vaporization within the crater formed by the direct beam and from relatively slow low-temperature pyrolysis around this crater.

Rapid devolatilization of small coal particles

Rapid devolatilization of small coal particles

Influence of electrical fields on intensity of combustion in shock-heated gases

Low-intensity electrical fields have been found to influence the combustion of clouds of small coal particles in shock-heated oxygen. The effects ranged from oscillations in the emission to the rapid quenching of emission. It is suggested that these effects are due to the production of multi-charged soot particles.

Microscope Observations of the Crushing of Coal

So far as I am aware, the peculiar effect described below of a disruptive pressure on small particles of coal has not hitherto been observed. The observations were made in the course of an examination of very fine coal dust from a ball mill which is claimed to grind coal to micron size.