Abstract
The physical, thermal, and chemical behavior of pulverized coal particles during thermal decomposition are examined for five coal types and two particle sizes for one of the bituminous coals. Particles were injected axially into a lean (35% excess air) methane/air flat flame with a nominal peak temperature of 1750°K. The significant events observed are classified by three time scales. Particles heat to the gas temperature in less than 10 msec, devolatilization occurs between 10 and 75 msec and, under the appropriate conditions, large soot particles are formed WRS and grow for times exceeding 75 msec. The events that accompany devolatilization are dependent upon coal type and particle size. For large bituminous particles (ca., 80 μm) a significant volatile fraction is ejected from the particle as a jet. This volatile jet reacts close to the particle producing a trail of small solid particles. The local heat released during the reaction of the volatiles, in combination with heterogeneous oxidation, increases the particle, temperature and raises it above that of the bulk gas stream. At later times, large soot structures, are formed which are attributed to the agglomeration of small, homogeneously formed soot on the volatile trail structures. Small bituminous particles (ca., 40 μm) burn with a higher intensity (i.e., higher temperature and more rapidly) with few trails and do not produce soot structures probably because of the more diffuse nature of the devolatilization process. Other ranks of coal exhibit different physical, thermal, and chemical behavior. For example, neither the lignites nor the anthracite produce volatile trials. Further, the particle temperature for the lignites is only slightly shifted above the bulk gas temperature in the devolatilization region while anthracite takes 50 msec to reach the bulk gas temperature level. This is attributable to the relatively low heat content of the volatiles in the former case and the low volatile content in the latter. The impact of the above observations on the formation of fuel NO is discussed.
Highlights
The design of pulverized coal combustors requires that the various phenomena occuring during thermal decomposition are understood in order to min~mfze pollutant emissions while providing optimum operating conditions
The results described in this paper were obtained in a study designed to examine the physical, thermal, and chemical behavior that accompany the thermal decomposition of pulverized coal particles
Experiments were conducted with the seven coal samples to examine the physical, temperature, and chemical characteristics associated with thermal decomposition of pulverized coal particles
Summary
The design of pulverized coal combustors requires that the various phenomena occuring during thermal decomposition are understood in order to min~mfze pollutant emissions while providing optimum operating conditions. The results described in this paper were obtained in a study designed to examine the physical, thermal, and chemical behavior that accompany the thermal decomposition of pulverized coal particles Such information is required to develop mechanistic models of thermal decomposition, and to provide data for numerical model development. The reactor (Fig. 1) was designed to satisfy two general criteria: (I) allow adequate control of the gaseous environmentsurrounding the coal particles, and (2) provide ready access for several physical and optical diagnostic systems, These criteria were satisfied by injecting the coal particles into the combustion products of a methane-air fired flat flame, stabilized on a 7.62 cm (3-inch) square sintered bronze burner with an embedded cooling coil, The flat flame was downfired within a 10.16 cm (4-inch) square stainless steel chimney in order to. Aerodynamic classification was employed to reduce the base size distribution into two narrow size classes with particle diameters of 40 + 4 ~m and 80 _+8 txm
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