Heating of the coal particles

Coal pyrolysis often leads to intensive formation of small and mostly spherical particles with sizes of 60-90 nm (Kozlova Kukhto, 1976 Shimansky, 1979). They originate from fast plasma heating of the coal particles and intensive formation of volatile compounds inside of them. This leads to an explosive decay of the coal particles into very small sizes. Another source of the small particles is the formation of soot in the plasma system from the preliminarily formed acetylene (Razina et al., 1973). Other factors affecting heating and pyrolysis of coal particles are the plasma-gas temperature and residence time of the... 

The surface of the coal particles oxidizes or weathers resulting in cracks, finer particles, and reduced agglomeration all of which may destroy coking properties. If spontaneous heating takes place, the calorific value of the coal is reduced. Hot spots must be carefully dug out and used as quickly as possible. Without spontaneous heating and with good compaction, calorific value losses below 1%/yr have been recorded. 

A simplified model of PC combustion includes the following sequence of events (I) on entering the furnace, a PC particle is heated rapidly, driving off the volatile components and leaving a char particle (2) the volatile components burn independently of the coal particle and (3) on completion of volatiles combustion, the remaining char particle burns. Whue this simple sequence may be generally correct, PC combustion is an extremely complex process involving many interrelated physical and chemical processes. 

The Soot Produced. The soot, which collected mainly on the inside of the metal container, was very finely divided optical and electron microscope studies indicated that most of the soot particles were submicron in size. The conversion of the coal particles of diameters originally up to 100m into this finely divided soot may well have resulted from a shattering of the coal particles since it is likely that a violent evolution of gas occurred under the extremely rapid heating. The analyses of the soots from three experiments with the 802... 

The coal bed from which this natural coke was formed has not been traced to an unaltered area. However, data reported previously indicated that the coal seams of this general area are in the high volatile range. This discrepancy between the reported volatility and the character of the coal particles in the coke may be a result of a relatively rapid increase in the stage of metamorphism immediately prior to coking of the coal by the heat from the igneous intrusion. 

In practice, the gases exiting the fluidized bed reactor contain a certain amount of ash and have to be cleaned. Also, the combustion products of coal are sometimes corrosive, which means that in addition to air being fed into the reactor, various other chemicals are added to ensure "clean" combustion products that will not corrode turbine blades or violate environmental standards. Coal combustion is a very active field of research, and many exciting developments are occurring there. In this analysis, we make certain assumptions that illustrate the thermodynamic concepts as clearly as possible. Therefore, we do not examine the effect of hydrodynamics, heat, and mass transfer, which are very important in the combustion of the coal particle and the distribution of combustion products. We do not expect that this will have a significant impact on the analysis. 

It is assumed that circulation of the cooling steam through the heat exchangers is started fifty seconds after feeding of the coal particles into the bed is initiated. The functional relationships among variables and the nominal values of various parameters employed in numerical computation are listed in Table I and II, respectively. Numerical simulation has been carried out separately for the type A combustor and the type B combuster in order to emphasize the differences between them. 

The release of volatile compounds starts in the H2 plasma when the temperature of the coal particles reaches about 1220K(Beiersetal., 1985). This critical temperature of volatiles formation is almost independent of the plasma-gas temperature. The temperatme of the coal particles remains fixed during the intensive generation of volatile compounds, which prevents effective heat exchange between the particles and the plasma gas. Coal-to-carbon conversion, therefore, also remains almost fixed at different plasma-gas temperatures. While C2H2 is a major product of plasma coal pyrolysis in Ar and H2, conventional by-products are... 

These two processes (i.e., devolatilization and char oxidation) may take place simultaneously, especially at very high heating rates. If devolatilization takes place in an oxidizing environment (e.g., air), then the fuel-rich gaseous and tar products react further in the gas phase to produce high temperatures in the vicinity of the coal particles. 

Ga.s-to-Pa.rticle Heat Transfer. Heat transfer between gas and particles is rapid because of the enormous particle surface area available. A Group A particle in a fluidized bed can be considered to have a uniform internal temperature. For Group B particles, particle temperature gradients occur in processes where rapid heat transfer occurs, such as in coal combustion. 

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