The thermal decomposition of pulverized coal particles

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 fiat flame with a nominal peak temperature of 1750~ 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 Izm) 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 Ixm) 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 trails. 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.

Thermally induced structural changes in coal combustion. Final report

“…They suggested that energy from the oxidizing volatiles was fed back to the coal panicles, increasing their temperature and thereby the volatile release rate and yield. An increase of the particle temperature due to the oxidation of volatiles was also reported by Seeker et al (1981). Devolatilization rates and yields increase with panicle temperature have also been observed in experiments by Kimber and Gray ( 1967) (1973) suggested that oxygen complex fonnation at the surface and char activation may also be important at this early stage of conversion.…”

Combustion Aerosols: Factors Governing Their Size and Composition and Implications to Human Health

“…[16][17][18][19][20] This is supported by simple treatments of nucleation and growth of particles in a boundary layer. [21][22][23] More detailed treatment of nucleation in the boundary layer of a growing particle is presented by Peshty et al, 24 who show that the correct treatment should allow for heat release due to condensation, which tends to suppress the nucleation rate locally.…”

Light Absorption by Carbonaceous Particles: An Investigative Review

“…Soot formation in idealized flames and heat transfer by soot in boilers were also hot topics in combustion research (McLean et al 1981;Seeker et al 1981), resulting in many measurements of pure soot. Concurrently, a series of field experiments investigated the light-absorbing component of atmospheric aerosol in Denver, Colorado .…”