OCR Text |
Show The method exhibits many of the features of the Monte Carlo method (Gibb, 1977) but is not hampered by the stochastic errors commonly a problem with that method. A fixed number of rays are emitted from each control surface. The ray is computationally followed and the radiant intensity is attenuated until another surface is struck; then the ray is terminated. As the rays are traced, the total incident energy is cataloged in each cell. Total energy deposited in each volume is accumulated for all rays penetrating a volume and can be written as shown in equation 33. Total radiant flux incident on each surface is accumulated for all rays striking a surface. Solutions are iterative and can be improved by increasing the number of rays emitted from each surface. The radiative transport equation requires absorption and scattering coefficients for the particle-gas mixture. Van De Hulst, (1957) discusses calculation of these coefficients for a particle mixture. In each computational cell the amount of each particle type is estimated (coal, char, ash or soot) and coefficients are computed using Mie theory (Van De Hulst, 1957; Dave, 1969). The absorption coefficients for the gas mixture are found using Edwards' wide-band model (Edwards, 1976). MODEL APPLICATION Pulverized-coal combustion was investigated for a 560-MW universal pressure utility boiler. The configuration of the furnace is shown in Figure 2. The dry ash furnace is opposed-wall fired with 20 circular-type pulverized-coal burners on each firing wall arranged in an array, 4 burners high by 5 burners wide. A burner, depicted in Figure 3, contains a central straight-tube "nozzle," 0.406 meter in diameter, through which a mixture of pulverized coal and air is discharged into the furnace at the burner throat. Secondary air (main combustion air) flows from the windbox through air swirl registers (in a clockwise direction) and enters the furnace through an annulus formed by the coal nozzle and the burner throat (1.04 meter in diameter). An impeller at the end of each coal nozzle deflects the coal into the swirling secondary airstream near the burner throat where combustion begins. Pulverized coal is supplied to the burners by ball and race type contrarotating pulverizers. At full load, a high volatile bituminous coal is supplied at a rate of 54.18 kg/s with 20% excess air. The characteristics of the pulverized coal are shown in Table 3. -16- |