On the temperature and reaction rate of burning pulverized fuels

Abstract

A two-color pyrometer technique has been used to measure the temperature of burningpulverized fuel particles flowing in dilute suspension on the centerline of a laminar flow reactor. The transparent reactor is fed by a premixed flat flame fueled with CH 4 −H 2 −O 2 −N 2 mixtures, which permit widely variable post-flame temperatures and oxygen concentrations to be obtained. The two-color system employs an optical-electronic system which includes two monochromators to acquire light emission intensity at two wavelengths simultaneously from individual burning particles passing through a 1 mm square area on the centerline of the reactor. Planck's law and the gray body assumption (verified here by measurements) are used to compute particle temperatures, and the average temperature is then determined by averaging the values for some 500 individual particles. Particle temperatures have been determined for various gas temperatures over a range ofoxygen mole fractions from 0 to 0.2 for size-graded (90 μm mean diameter) samples of a petroleum coke and a flash pyrolysis char. Particle burning rates per unit external surface area were determined from the measured temperatures by a heat balance analysis. The functional variation in burning rate with temperature was then used to infer whether the burning rate was controlled by chemical rates (Zone I), the combined effects of chemistry and pore diffusion (Zone II), or bulk diffusion (Zone III). For the petroleum coke, both zone I and Zone II conditions were observed, and chemical rate coefficients for each zone were derived. For the char, Zone II and a transition to Zone III conditions were observed, and a chemical rate coefficient for Zone II was derived. Reaction orders and activation energies were in the range expected from previous oxidation studies, and the chemical rates for both samples were in good agreement with previous measurements by a different method in another laboratory. The results show that careful temperature measurements can provide a sensitive method for the determination of pulverized fuel burning rates.