Domains of flammability and thermal ignitability for pulverized coals and other dusts: Particle size dependences and microscopic residue analyses

Abstract

The particle size dependence for the lean limits of flammability in air of Pocahontas coal dust (16% as received volatility, by ASTM proximate analysis), Pittsburgh coal dust (35% volatility), and polyethylene powder (100% volatility) were measured using narrow size distributions with average diameters ranging from 2 μm to over 400 μm. In all cases, the lean limits measured in an 8-liter chamber became insensitive to particle size below some characteristic diameter. Above those characteristic diameters, the lean limit concentrations increased significantly with increasing particle size until a critical size was reached, above which the dust was nonflammable for any concentration at ambient temperature and pressure. Both the characteristic diameter and the coarse size limit of flammability (critical diameter) were observed to increase monotonically with increasing dust volatility and increasing oxygen content of the dispersing gas. The same narrow size distributions were also used to measure the particle size dependenceof the thermal ignitability of those same dusts dispersed into preheated air in a 1-liter furnace. The minimum autoignition temperatures (AIT's) display behavior similar to that of the lean limit concentrations, except that heating of the mixture to the elevated temperature required for such thermal ignitability studies markedly increases its flammability range. At those elevated initial temperatures, the characteristic sizes are shifted to larger diameters and no critical sizes were observed for some of the AIT measurements even at the coarsest sizes studied. Also presented are scanning electron microscope (SEM) data that reveal the structuralchanges in coal particles that resulted from their participation in dust explosions. Char residues that are pockmarked with blow holes are the typical remnants from all dust explosions of bituminous coals. At high dust concentrations, those char residues are fused into large, agglomerated masses. Preignition and postignition residues sampled from the thermal ignitability furnace show the same pockmarked appearance for their char residues. In another experiment, using a high power, pulsed CO 2 laser beam, it was possible to carefully control the pyrolysis exposure time and to obtain SEM photomicrographs during the early stages of the pyrolysis-devolatilization process. A theory is presented which isolates the important physical processes in their simplestform. The processes are: (1) heating and devolatilization of the dust particles, (2) mixing of those volatiles with air, and (3) gas-phase combustion of the volatiles-air mixture. There are two loss processes that compete with that propagation sequence and can therefore quench propagation: (a) natural convection or buoyancy, operating through the mechanism of flame stretch, and (b) conductive-convective heat losses to dust particles, which, in addition to being sources of needed fuel, are also heat sinks. The measured insensitivity of the lean limit concentrations to particle size for the finer dusts reflects propagation under rate control by process 3 and quenching by loss process a. A particle size dependence appears at the characteristic diameter because rate control of propagation is shifting from process 3 to process 1, while quenching is still by loss process a. The dust mixture is nonflammable above some critical diameter when the propagation process is entirely rate controlled by process 1 and quenching occurs by a combination of loss processes a and b.