| OCR Text |
Show d2 = d 2 - At (5-4) o where A., the evaporation coefficient, represents a value for evaporation conditions or near combustion conditions, depending on the volatility of the droplet. According to calculations, this period of rapid evaporation to the diameter at which coke particles are exposed is very brief, on the order of 1 to 2 milliseconds (for a 200 micron slurry droplet loaded with 45% by weight coke and A = 40 x 10 3 (cm2/sec). However, during this period the coke particles are prevented from heating to ignition due to "cooling effect" of evaporation. Also, during this fuel evaporation period, according to Wood, et al (Ref. 17), and Law (Ref. 18), droplet instability and breakup may also occur as lighter components vaporize from the droplet. After this brief droplet evaporation period, coke particle surface areas are exposed to the combustion environment and diffusion and/or kinetic- limited burning of particles commences. Depending on the particle (or agglomerate) size, temperature and carbon reactivity, a flame sheet either surrounds the particle at a distance from the particle or is contiguous to it. Diffusion-limited reaction occurs with the flame sheet detached from the coke particle surface and is dominant when the coke weight losses are high (i.e. high particle temperature, reactivity and large particle size). Kinetic-limited reactions occur when the "flame sheet" is on the particle surface and is dominant for lower particle temperatures and smaller sizes. Several mathematical treatments of the various stages of particle combustion based on this simplified model have correlated well with experment (Refs. 19, 20). Several investigations (Refs. 21, 22, 23, 24) have developed global reaction rates of petroleum cokes, coals, chars and carbons for different particle sizes and temperatures to simplify the determination of burning rates. Usually these take the form of the Arrhenius law and are expressed as: r = A exp [-Ea/RT ] (5-5) 19-27 |
