OCR Text |
Show applied to carbon loss from boilers firing pulverized coal (Walsh et al., 1994). The method is to calculate the size distribution of char particles formed on devolatilization of the coal feed, then estimate the extent of burnout of each size of char particles during the residence time above the burners, beginning with the oxygen remaining after volatiles combustion. Particles are assumed to bum at constant size in gas having uniform temperature throughout the postflame region. Particle temperatures are equated to the gas temperature, and the gas temperature is estimated from the adiabatic flame and furnace exit gas temperatures (Hottel and Sarofim, 1967). The present estimates of gas temperature range from 1600 K (40% excess air) to 1760 K (stoichiometric). The model adopted here is based on the observation of relatively slow fluctuations in oxygen mole fraction in the upper furnace (Figure 4). The average upward velocity of furnace gas is 8 mls. A frequency of 1 Hz for the fluctuations in oxygen therefore corresponds to a length scale of 8 m for the concentration changes. The fluctuations in oxygen are evidently associated with slow variations in fuel/air ratio and large-scale unmixedness of fuel and air from the burner or burners whose flow was sampled. This suggested that the flow in the postflame region might be successfully treated as consisting of segregated parcels of gas, each parcel containing a fixed concentration of carbon (solid plus oxides) and an air/fuel ratio which remains unchanged during its passage from the end of the volatiles flame to the furnace exit, without any mixing. The shape of the distribution of excess air observed at a single point in the furnace (Figure 5) indicates that a normal distribution is a useful approximation to the probability density function for excess air in the segregated flow, although the shape of the distribution might be different if the oxygen mole fractions were measured at a representative set of locations over the cross section of the furnace. Large variations in the spatial distribution of average oxygen, arising from air inleakage and burner imbalance in coal-fired boilers, were observed by Thompson et al. (1994). The oxygen in each parcel of gas and particles was assumed to decay according to a second-order rate expression describing the char-oxygen reaction, from its initial level on completion of volatiles combustion toward the value corresponding to complete burnout of char. In the model, oxygen and carbon consumption are not directly coupled; the oxygen 12 |