| OCR Text |
Show 4 of simultaneous imaging of reflected light and near infrared (1R) emission. The IR images can be used for determination of radiance temperatures. t VARIATION OF CHAR REACTIVITY DURING COMBUSTION High temperature char combustion rates have most often been described by global kinetic expressions in convenient mathematical forms that assume a constant char reactivity [Smith, 1982; Essenhigh, 1981]. While this approach has been successfully applied to laboratory combustion data in the early and intermediate stages of combustion, its ability to predict carbon burnout to levels that will result in an operationally acceptable LOI (i.e., carbon conversion > 99%) is questionable because of the phenomena described herein. General Features of Reactivity Loss Few studies have focused on the late stages of char combustion under carefully controlled conditions. Among those, Vleeskens and Nandi [Vleeskens and Nandi, 1986] have investigated the fuel-related factors detennining the extent of coal burnout in a drop tube furnace. Mitchell [Mitchell, 1990] observed numerous particles with low temperatures at high carbon conversion, and identified them as highly-reacted, high-inorganic-content char particles. In the present investigation, a thorough description of char reactivity evolution during combustion is accomplished using the two systems described previously. Image sequences were obtained for 20 to 100 particles of lllinois No.6, Pittsburgh No.8, and Dietz coal chars, Beulah Lignite char, and Pine and Switchgrass biomass chars. Dual images of reflected visible light and near-infrared emission were obtained for selected particles. A typical dual-image sequence for a bituminous coal char is shown in Figure 1, along with time-resolved radiance temperatures determined from digitization of the near-infrared images. The combined data and images display a period of bright incandescence from 0.8 to 1.2 seconds, followed by a relatively abrupt drop in temperature of 125 K and a long, slow, nearly-isothermal, fmal burnout to a carbon-free ash particle. The gradual decrease in radiance temperature late in the particle lifetime is due in part to decreases in emissivity, but the initial rapid decrease reflects a decrease in actual temperature, since it is large, and occurs abruptly while the particles are still optically dark. These t Radiance temperature is defined as the temperature of a hypothetical black body emitting the same radiative power as the real object (particle) in the wavelength range of interest (her 700 - 1000 nm). For particles that have undergone low to intermediate extents of burnout, emissivities are approximately 0.8 [Baxter et al., 1988] and true particle temperatures will be approximately 20 K greater than the reported radiance temperatures. |
