OCR Text |
Show coating the inside and outside of the closed end of the tube with platinum. The electromotive force of the cell, described by the Nernst equation, is proportional to the logarithm of the ratio of the mole fractions of oxygen in reference air and furnace gas, on the inside and outside of the zirconia tube, respectively. A thermocouple measures the average cell temperature, needed to convert the potential difference to oxygen concentration or mole fraction. A sample of the output from the probe is shown in Figure 4, at a location in the upper furnace, opposite the arch, 6.4 m above the centers of the top row of burners, 0.5 m into the furnace from the middle of the burner wall. During the short period shown, oxygen is seen to vary from roughly 1.5 to 4.5 mol%. The fluctuations occur at a wide range of frequencies, the highest being of order 1 Hz, and the lowest visible in the figure of order 0.01 Hz. Air/fuel ratios were calculated from the instantaneous oxygen mole fractions. Analysis of data collected over a period of 30 minutes produced a distribution of excess air which was approximately normal, as shown in Figure 5. Measurements of gas temperature at the exit of the furnace have been made using a suction pyrometer (R. E. Douglas, CONSOL, Inc., personal communication) at high and low steam flowrates. Interpolation of these measurements yields an estimate of 1400 K (20600F) for the average furnace exit gas temperature during the test under consideration. This temperature is much higher than the value of 1020 K (1376°F) used in a previous analysis of the data (Walsh et al., 1994). As a result of air inleakage, the apparent excess air depends upon the sampling location at which oxygen is measured. The average excess air based on the control room oxygen record was 27.1 %. The value obtained from the average oxygen observed by the mobile continuous emissions monitor brought in for the tests was 27.8% (PENELEC, 1992), and at the platinum/zirconia probe in the upper furnace, 0.5 m from the wall, it was 21 % (Figure 5). CALCULATIONS AND RESULTS A convenient set of correlations was developed by the Babcock & Wilcox Company (1963) for the estimation of carbon loss from its pulverized coal-fired boilers. The results applicable to dry-ash-removal furnaces are shown in Figure 6. The upper curves are first 7 |