Large Coal Particles Research Articles

A study on devolatilization of large coal particles

A generalized devolatilization model for large coal particles is proposed in this paper. The difference between the present model and previous ones is that the equivalent activation energy, E, and the equivalent frequency factor, K, of coal particle devolatilization are independent of coal type, and depend only on the final temperature, T ∞, of the coal particles. The relationship between E K and T ∞ has been obtained and is found to be universal for all coal types. If the final volatile yield, V ∞, is determined, the devolatilization processes of a large coal particle for any kind of coal can be predicted by using the present model. This model has been tested with five Chinese coals with quite different properties, diameters ranging from 3 mm to 9 mm, and temperatures ranging from 1173K to 1733K. The calculated results agree with experimental data.

Measurements of Surface Combustion Rate of Single Coal Particles in Laminar Flow Furnace

Abstract Study of The pulverised coal combustion in air was performed using a laminar flow furnace at gas temperatures between 800 and 1200° C, particle size between 44 and 359 μ m and oxygen level of 21 percent. The time sequential variation of the fractional burn-off, the particle size distribution and the particle density during the pulverized coal combustion were measured by using two different type coals. The curve of the fractional bum-off indicated that the combustion of the pulverized coal is a two-staged combustion — the rapid volatile matter combustion followed by the slow char combustion. However, it was observed that char combustion occured simultaneously in the volatile matter combustion region. From the measurements of the combustion rate based on the unit external surface area of the particle, the rate of large coal particle was higher than that of small ones under the same gas temperature within the range of particle size used in this experiment. The fact could be explained by the temperat...

A study on atomization of coal water mixture. (2nd report. Atomizing mechanism).

In order to elucidate the atomizing mechanism of Coal Water Mixture (CWM), several experiments on the atomization of CWM will be carried out with twin fluid atomizers. The effect of coal sizes on the formation of CWM spray droplets is investigated in detail. It is also found that large droplets of CWM spray consist of numerous fine coal particles and that large coal particles do not form Nucli of large spray droplets. From these experimental results, the breakup mechanism of CWM uneer high shear stress by gas-blasting at the exitport of the atomizer is proposed.

A model for the devolatilization of large coal particles

A model for the devolatilization of large coal particles

Distributed kinetic parameters for methane evolution during coal pyrolysis

The disparity between reported kinetic parameters for overall coal decomposition as well as volatile species evolution has been attributed to differences in the type of coal being studied as well as in the experimental technique employed. In this study, data reported for methane evolution in fluidized beds are considered and are found to be largely insensitive to coal type. Additionally, distributed kinetic parameters obtained using the heated grid, entrained flow and slow heating techniques are found to represent the fluidized bed data for both pulverized and large coal particles, provided that the transport mechanisms are also taken into account.

The Fate of Fuel Nitrogen in Coal-Fired Spreader Stokers

Abstract Abstract-Coal-fired spreader-stoker boilers are recognized as significant stationary sources of nitrogen oxide (NOx) emissions. Abatement of NOx emissions from spreader stokers has received relatively little research attention. The purpo se of this investigation was to assess the role of first-stage stoichiometry and local thermal environment in the fate of fuel nitrogen. The research was specifically concerned with identifying nitrogenous species evolution and reaction in the fuel-bed burning regime. The approach was experimental, utilizing a fixed-bed furnace configured to simulate the bed-phase combustion environment of coal-fired spreader stokers. Large coal particles sized from 6 through 32mm were fed continuously onto a fixed retort forming a relatively thin (lO to 15em) fuel bed. First-stage stoichiometry was the primary combustion parameter influencing NO formation. Substantial reductions in exhaust NO emissions were achieved under staged combustion conditions. In-flame measurements indic...

Devolatilization of large coal particles in fluidized beds

Analysis of devolatilization of predried large coal particles in fluidized beds requires consideration of both the chemical kinetics of coal decomposition and transport processes. Models available either assume the devolatilization particle to be isothermal (whereas it may be shown that, in general, large temperature gradients may exist within the particle) or require extensive numerical integration procedures. This Paper describes a model which permits formulation of analytical and easy-to-use equations for the estimation of the devolatilization history of a large predried coal particle in a fluidized bed. The model predictions are compared with experimental data collected for Mississippi lignite. A correlation is proposed for the estimation of the total devolatilization time. The analytical solutions presented may be used with ease in coupling the devolatilization process to the other phenomena, such as drying and/or combustion of volatiles and residual char, occuring during fluidized bed combustion of coal.

Release of volatiles from large coal particles in a hot fluidized bed

Coal particles with diameters of 3–11 mm were injected into a small, hot bed of sand fluidized by nitrogen. Volatiles evolution was followed by sampling the exit gas stream and subsequent analysis by gas chromatography. Three Australian coals covering a range of volatile matter were studied and the effects of coal particle size and bed temperature were determined. The yields of gaseous components, char and tar are explained by consideration of the competitive reactions for coal hydrogen and oxygen and secondary reactions of the volatile species within the coal particle. The pore structure developed during devolatilization has a significant effect on the extent of these secondary reactions. It is concluded that heat transfer is the main process controlling the volatilization time in fluidized bed combustors. The time required for heat transfer into the coal particle, determined by calculation and experiment, agrees with the measured volatilization time. Significant factors are external heat transfer to the surface of the particle, internal conduction through the coal substance and radiation through the pores, and the counterflow of volatiles out of the coal particle. For different coals, variations in the volatilization time appear to be caused by the development of different pore structures, which affect radiant heat transfer through the pores.

Plume model for large particle fluidized-bed combustors

A model is proposed to represent the underfeed, fluidized coal combustor. Its assumptions are described and its mathematical representation developed. This model accounts for the possible rapid release of volatiles near the feed ports, particle reaction and shrinkage in the bed, elutriation of unburned fines and afterburning of volatiles above the bed. Design charts are then presented to predict the carbon efficiency of the combustor as well as the temperature jump above the bed in terms of the type of coal used, the number of feed ports in the bed, percentage excess air, gas velocity and the amount of secondary air needed to introduce the coal. As a special case, this model also represents the fluidized combustor with an overfeed of large coal particles or any combination of overfeed of large particles and underfeed of fines.

Pyrite oxidation in aqueous ferric chloride

AbstractThe rate of pyrite oxidation in aqueous ferric chloride was determined for two distinct solid particle systems: industrial grade pyrite and coal particles containing pyrite. The oxidation rate for the pyrite particle system was found to increase significantly with increasing temperature (40° to 100°C), ferric chloride concentration (0.1 and 1.0 M), and pyrite loading (2 to 20 g/l); the rate decreased with increasing particle size (−325 to 140 mesh). Agitation did not have a significant effect, and a kinetic model was developed and fit to the experimental data.For the coal particle system used in this study, the most important variable was particle size. The oxidation rate of pyrite in coal smaller than 325 mesh was much greater than in larger coal particles. The effect of temperature (80° to 100°C) on the oxidation of pyrite in coal was not significant, nor was the effect of pretreatment with 0.1N hydrochloric acid. Approximately half of the detected ferric iron reduction was attributable to pyrite oxidation; the balance arises from other coal reactions.