H ShD

where hm is the mass flux of oxygen per unit area of surface per unit of concentration difference between that at the surface and that in the gas outside the boundary layer (m/sec), Sh is the Sherwood number (dimensionless), and D is the diffusion coefficient of oxygen through the gas mixture surrounding the particle (m2/sec) [15].

Equation (5-15) can be used to derive the combustion rate q, in units of mass of carbon oxidized per unit time per unit area of particle outer surface:

where D is evaluated at a mean temperature Tm (K) in the diffusion layer, Ps is the oxygen partial pressure at the particle surface (kN/m2), and the universal gas constant R has the units 8.31 J/mol K.

The chemical kinetic rate coefficient, kc, is expressed by an Arrhenius-type equation:

where Aa is the apparent rate constant based on particle outer surface area (kg/m2s per kN/m2 of partial pressure of oxygen), and Ea is the apparent activation energy for Regime II combustion (kJ kg/mol).

Coal Combustion Systems

The manner in which coal is burned and the devices in which it is burned are primarily determined by the desired unit size or capacity (i.e., required hourly steam production or electricity generation) and coal type and quality. The combustion methods, fixed-bed (i.e., stokers), fluidized-bed, and suspension firing are discussed.

IffSi Raw fuel

Jiff

WM Burning fuel

(iSV" Ash

Air flow Ignition plane r'i I: J rU

FIGURE 5-9. Patterns of feeding coal and combustion air to stokers: (a) overfeed; (b) underfeed; (c) crossfeed. (From Elliot, M. A., Ed., Chemistry of Coal Utilization, Secondary Suppl. Vol., John Wiley & Sons, New York, 1981. With permission.)

Fixed-Bed Combustion

Fixed-bed combustion covers a wide variety of applications, including domestic space heaters, underground gasification, and industrial stokers. It is the latter that are of interest for steam and power generation and are discussed here. Stokers were used to burn coal as early as the 1700s [7,14]. Stokers have evolved over the years from simple design to quite sophisticated devices to burn a variety of fuels including coal. Stokers are generally divided into three general groups, depending on how the fuel reaches the grate (i.e., surface that contains the coal and allows combustion air to be introduced into the fuel bed) of the stoker for burning: underfeed, overfeed, and spreader designs. Three patterns of feeding coal and combustion air have been developed and are used singly or in combination in commercial equipment [2]. These patterns, illustrated in Figure 5-9, are referred to as:

• Overfeed—The fuel is fed onto the top of the bed and flows down as it is consumed while combustion air flows up through successive layers of ash, incandescent coke, and fresh coal;

• Underfeed—The flows of coal and combustion are parallel and usually upward;

• Crossfeed—The fuel moves horizontally and the combustion air moves upward at right angles to the fuel.

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