OCR Text |
Show The PCSV -Pis an in situ. instrument capable of measuring the number density, size distribution, and speed of dIsperse particles. Particle sizes are inferred from the amplitude of the near-forward scatt~red He-Ne laser beam. In this application two laser beams were used separately, one WIth a 25 flm beam waist for the particles in the size range of 0.4-2.0 flm ~d the second with a 250 flm beam waist for particles in the size range 2.0-140 flm. LIght scattered from the particles passing through the sample volume of the beam waist is cO.llected over a solid angle and monitored with a photomultiplier tube. The size and trajectory of the particles affect the maximum amplitude of the detected pulses in known ways. As long as the particles are small compared to the beam waist, Mie theory predicts their size dependence. Trajectory dependencies are calculated by a statistical technique based on the following assumptions: 1) the average flux of particles of a given diameter does not vary spatially over the width of the beam and 2) a large number of particles of each size are detected. U sing this statistical technique, particle size distributions are calculated from the measured frequencies of pulses of each amplitude. Individual pulse durations are related to the known width of the beam waist and are used to calculate particle speeds (Bonin, 1992; Bonin & Queiroz, 1991; Holve, 1980; Holve, 1982; Holve, 1986; Holve & Self, 1979). The specifications of the industrial boiler are given in Table 1. It contains 161 m2 of effective heating surface with a furnace heat release of 517 kW 1m3, which is representative of this class of D-type boilers. The mean gas residence time in the boiler is less than one second when firing natural gas. Additional information on this boiler can be found elsewhere (Sharifi, 1996). Figure 2 illustrates the boiler, the fuel delivery system and the baghouse. The three observation ports used to provide access to a horizontal slice of the boiler, located at a distance equal to half of the height of the boiler, are shown on the side wall of the boiler. The locations of these observation points correspond to xID=3.75, 8.75 and 14.25, where x is the axial distance on the burnerlboiler centerline and D is the burner diameter. Table 1: Boiler Specifications Rated capacity 6,800 kg/h saturated steam at 2 bar (490 K) Type D-type water tube Designed fuel Fuel oil and natural gas Retrofitted fuels Micronized coal and Coal Water Slurry Fuel Thennal input 4,400-5,600 kW Results and Discussion: Measurements of Gas Temperature As mentioned previously, the measurements of gas temperature, vel<;>city, num~er density, particle size distribution and speed were taken for two sets of boIler operatIng conditions. Table 2 provides a summary of the boiler operating conditions and performance data for each test. Measurements of gas temperature by suction pyrometry, for Test 1, are depicted in Figures 3-5. The highest measurement of the mean gas temperature (1,655 K), was taken at xID=8.75 (Figure 4). ~ased on the. temperature gradient (Figures 3 and 4) and considering the fact th~t pulvenzed coal partIcl~s ~ndergo ignition and devolatilization within 1 to 2 burner dIameters of the burner tIP In most industrial pulverized coal burners, it can be stated that ignition .and devolat~lization of the micronized coal particles have been almost complete~ by ~he tIme t~e partIcles reach the axial location of xID=8.75. Consequently, the sbght Increase In the values of gas temperature from xID=3.5 (Figure 3) to x/D=8.75 (Figure 4) should be due to the combustion of char. |