Burning coal particles

Catalyst regeneration in a spherical particle (burning coal particle) can be represented by the following dimensionless equations) ]... 

In a combustion chamber, soot volume fraction or number densities are usually not known. It is generally accepted that the primary soot particles are spherical in shape and about 20-60 nm in diameter (see the review by Charalampopoulos [185]). However, depending on the flow and combustion characteristics of the system, they agglomerate to form irregularly shaped large particles. The shape can be clusters of spheres or cylindrical long tails attached to burning coal particles. 

Joutsenoja T, Heino P, Hernberg R. Pyrometric temperature and size measurements of burning coal particles in a fluidized bed combustion reactor. Combust Flame 118 707-717, 1999. 

Burning times for coal particles are obtained from integrated reaction rates. For larger particles (>100 fim) and at practical combustion temperatures, there is a good correlation between theory and experiment for char burnout. Experimental data are found to obey the Nusselt "square law" which states that the burning time varies with the square of the initial particle diameter (t ). However, for particle sizes smaller than 100 p.m, the Nusselt... 

Fluidized combustion of coal entails the burning of coal particles in a hot fluidized bed of noncombustible particles, usually a mixture of ash and limestone. Once the coal is fed into the bed it is rapidly dispersed throughout the bed as it bums. The bed temperature is controUed by means of heat exchanger tubes. Elutriation is responsible for the removal of the smallest soHd particles and the larger soHd particles are removed through bed drain pipes. To increase combustion efficiency the particles elutriated from the bed are coUected in a cyclone and are either re-injected into the main bed or burned in a separate bed operated at lower fluidizing velocity and higher temperature. 

The radiation from a flame is due to radiation from burning soot particles of microscopic andsubmicroscopic dimensions, from suspended larger particles of coal, coke, or ash, and from the water vapor and carbon dioxide in the hot gaseous combustion products. The contribution of radiation emitted by the combustion process itself, so-called chemiluminescence, is relatively neghgible. Common to these problems is the effect of the shape of the emitting volume on the radiative fliix this is considered first. 

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

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. 

Typically, approximately 30 to 50 percent of the coal is burned in suspension. It excessive fines are present, more coal particles will be carried out of the furnace and burned in suspension, and veiy little ash will be available to provide a protective cover for the grate surface. On the other hand, if sufficient fines are not present, not all the fuel will... 

An important principle is that the coal becomes ignited from the fuel which is on top of it and further down the grate. This reduces carry-over of smalTunburned particles, as the burning coal on top filters them out. Volatiles released from the fresh coal are also ignited and consumed in the burning layer. This minimizes smoke formation which is caused by incomplete combustion of the volatiles. 

The high temperatures of coal char oxidation lead to a partial vaporization of the mineral or ash inclusions. Compounds of the alkali metals, the alkaline earth metals, silicon, and iron are volatilized during char combustion. The volatilization of silicon, magnesium, calcium, and iron can be greatly enhanced by reduction of their refractory oxides to more volatile forms (e.g., metal suboxides or elemental metals) in the locally reducing environment of the coal particle. The volatilized suboxides and elemental metals are then reoxidized in the boundary layer around the burning particle, where they subsequently nucleate to form a submicron aerosol. 

CycUme Furnaces In cyclone firing (Fig. 24-l4d) the coal is not pulverized but is crushed to 4-mesh (4.76-mm) size and admitted tangentially with primary air to a horizontal cylindrical chamber, called a cydone furnace, which is connected peripherally to a boiler furnace. Secondary air also is admitted, so that most all of the coal bums within the chamber. The combustion gas then flows into the boiler furnace. In the cyclone furnace, finer coal particles burn in suspension and the... 

Typically, approximately 30 to 50 percent of the coal is burned in suspension. If excessive fines are present, more coal particles will be... 

In a suspended bed or entrained flow reactor technology, the coal is crushed, dried, and then pulverized to fine powder in a crusher and mill. As Table 9.1 shows, the coal particles used in entrained flow reactors are very small. The pulverized coal is transported with air to the furnace (primary air), and secondary air is heated and fed into the combustor to ensure complete combustion. The residence time of the coal in the furnace is typically around 1-2 s, which usually suffices for complete combustion. However, not all coal burns completely, and fly ash will be generated (see Table 9.1). 

Figure 2-1.10 Small coal particles are burned in suspension, but larger particles are forced against the outer wall. The resulting slag is mostly liquid because of the high radiant temperature and low fusion temperature, and is drained from the bottom of the furnace through a tap.6 Cyclone furnaces are most common in utility and large industrial applications.

The process of particle combustion depends on the physical and chemical nature of the solid as it heats and burns. Coal is a complex material of volatile and nonvolatile components which becomes increasingly porous during volatilization of low-boiling constituents in burning. The crucial practical questions for boiler design concern whether pulverized fuel combustion is controlled by oxidizer diffusion or by chemical kinetics. 

Figure 8. Time-resolved values of the fractional weight retention of coal particles initially 3 mm in diameter burned in a fluidized bed at 1173 K and an oxygen concentration of 5 percent. Solid line calculated from theory, data points experimental.

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