COMBUSTION OF CWF AGGLOMERATES FROM PULVERIZED OR MICRONIZED BITUMINOUS COAL, CARBON-BLACK, AND DIESEL SOOT

Fundamental studies were conducted to explore the role of coal grind size on the combustion characteristics of coal-water fuel (CWF) agglomerates. The agglomerates were prepared from bituminous coals of two different grind sizes: pulverized (40 mum mean size), and micronized and beneficiated (4 mum mean size) as well as blends thereof. Experiments involved single predried agglomerates, free-falling in a drop-tube laminar-flow furnace at high heating rates. Combustion took place in oxygen partial pressures of 10% or 21% at a furnace gas temperature of 1450 K. Additional experiments were performed with predried agglomerates from water slurries of carbon black or diesel soot (both of particulate size and aggregate sizes in the order of 0.050 and 0.5 mum, respectively). All agglomerates were in the size range of 150-900 mum and were either plain or impregnated with calcium magnesium acetate (CMA). Upon heating and devolatilization, the bituminous coal agglomerates studied were found to melt, mildly swell, and form cenospheric structures. Individual pulverized coal particles also formed small cenospheres themselves, superimposed on large agglomerate-derived cenospheres. Combustion of CWF agglomerates, in the size range examined, occurred with distinct volatile and char combustion phases with the latter burning in a diffusion-controlled mode. The combustion behavior of pulverized and micronized coal agglomerates of the same size was strikingly similar. The presence of dissolved calcium magnesium acetate (CMA) accelerated setting of the slurries and, upon water evaporation it suppressed melting and cenosphere formation of the coal particles and agglomerates alike. Also, CMA mildly influenced the agglomerate combustion behavior, causing both larger volatile flames and char temperatures that peaked at the first half of the burnout period and decreased thereafter, but did not substantially affect the char burnout time. Carbon black and diesel soot agglomerates did not form cenospheres and the latter burned a little hotter and faster but, basically, their combustion was similar to that of CWF chars. Bulk fragmentation was consistently observed only in the case of CMA-impregnated carbon black agglomerates.