| OCR Text |
Show Shown in Figure 5-4 is a proposed model of various stages of slurry fuel combustion. Thus, the following steps are shown: Slurry droplet heatup. Droplet evaporation (lightest petroleum fractions first). Droplet instability and breakup. Exposure of coke particle surface area in a coke-oil agglomerate to combustion environment. Particle/agglomerate ignition. Diffusion-limited particle/agglomerate oxidation. Agglomerate breakup. Pore diffusion and surface chemical rate limited oxidation According to our model, the size of the agglomerate is dependent on primarily the size of the original droplet formed during atomization. Agglomerate breakup may be dependent on: (1) volatile content of the particles, (2) heat-up rate of the particles in the agglomerate, and (3) the uncombusted oil acting as a "binder". Minimizing the size and maximizing the porosity or surface area of the agglomerate ensures rapid burnout and is achieved with efficient atomization and originally small coke particle sizes. The time required for each successive phase of the slurry droplet combustion process can be estimated by droplet evaporation and solid particle flame sheet models (Refs. 14, 15, 16). Depending on the reactivity of the solid particle, roughly one to two orders-of-magnitude increases in time are required to burn a solid particle, such as coke, when compared to an equivalent liquid fuel droplet diameter. According to this model, during the initial phase of the slurry combustion prcess, lighter components of the residual oil are burned with a resulting weight loss of the droplet. This droplet "burning", under a convective flow field provided by an actual combustion environment, closely results in the evaporation of the droplet at fuel wet-bulb conditions. Actual vaporization of the light components to the hot gaseous environment follows the so-called "d2 law" to accurately correlate droplet lifetime until exposure and heat-up of the petroleum coke particles and agglomerate, that is: 19-25 |
