| The behavior of non-recirculating pulverized coal flames has been studied using a refractory tunnel combustor which was originally designed for the purpose of obtaining one-dimensional profiles for kinetic determinations. In this combustor, the flame-front (ignition front) stabilizes inside the tunnel, downstream of a water-cooled tube bank. The observed flames were laminar, but one-dimensional structures (with a flat flame-front) were only observed for coals with a 50% volume diameter of 30 microns or less, at air rates from 70 to 80% theoretical air. For stoichiometric firing conditions and standard grinds of coal, the flame-front took on an inverted conical shape.;The three-dimensional nature of the flames for normal firing conditions is due to the combined influences of the dust concentration and the inlet velocity profile, coupled with the system design. The flame-front stabilizes close to the tube bank near the combustor walls and farther from the tube bank near the combustor axis due to the incoming velocity profile. The flame speed varies along the flame-front, although the fundamental flame speed is constant for the mixture. The flame speed is highest near the combustor axis and lowest near the walls. This is attributed to higher radiant energy losses from reacting streams near the walls due to the proximity of that region of the flame-front to the tube bank. Conduction, volatile matter release, and diffusion may play more important roles in flame stabilization in the boundary layer regions.;As particle size decreases and fuel/air ratio increases, the absorption coefficient of the incoming cloud becomes high enough such that ignition occurs much closer to the tube bank, and thus, the variation in ignition distance is greatly reduced. For these conditions, the flame-front is nearly flat and the structure is approximately one-dimensional. This is supported by results of transverse (cross-flow) measurements for a range of coals, firing conditions, and flow conditions. Further support is provided by analytical predictions which show nearly flat flame-fronts for small particle sizes and air deficient conditions, but conical flame-fronts otherwise. |
