OCR Text |
Show 0.25 and 1.0 m (figs. 2 to 5). The rate of char combustion in this region of near zero oxygen concentration will be extremely slow, and the lack of volatile matter remaining in the char prevents further weight loss from the coal particles, until oxygen becomes locally available for char and soot combustion. Oxygen becomes available between 1.0 and 1.5 m, after the combustion of expelled volatile matter is completed, and results in the relatively slow rate loss of solid material from heterogeneous combustion occurring typically after 1.5 m, (see fig. 1). The burnout levels measured at 0.25 m (45% - 65%) for the bituminous coals are much higher than the proximate volatile matter (19 - 38%) in the input coals, but it is believed that this burnout is nevertheless primarily a result of devolatilization. The extremely high particle heating rates (IO6 C/S) and high particle temperatures [8] that should be produced by mixing and combustion in the 200 m/s, hot blast should considerably enhance the proximate volatile matter yield. Lower rank bituminous coals typically yield 65 - 70% volatile matter when subjected to conditions of high temperature and fast heating rates [5, 6J. Another important factor in support of the burnout for distances less than 1.0 m being primarily a result of pyrolysis, is the short residence time of particles in the blast air, approximately 2 ms, before volatile ignition and combustion consumes the locally available oxygen. During this 2 ms period the particles must first heat up to ignition temperature, reducing the initial time for char combustion even more. The largest fraction of soot and char combustion probably occurs after the completion of volatile combustion. The visual observation of the flames supported reasonably well this interpretation of the measurements. With a 1200°C blast temperature and a bituminous coal, the very bright initial flame region, which results from volatile combustion and soot formation, typically started well inside the tuyere and finished at between 1.0 and 1.5 m. In fig. IB the variation of blast temperature, particle size, and stoichiometric ratio, shows only a small effect on the pyrolysis controlled burnout. They exhibit an effect instead in the region of char and soot combustion. Here both particle size and stoichiometric ratio influence the combustion rate, with the stoichiometric ratio showing the more dominant effect. 10-10 |