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Show the form of flocculated soot particles with fractal geometries. This has also been observed previously for Pittsburgh#8 (see refs. 21 and 22). It seems to be a reasonable assumption that the majority of the tar is converted to soot. O n this basis the major source of the radiant field around a burning coal particle can be assumed to be pyrolysed tar, and if so, combustion rates can be readily calculated. 2.3 Char Formation. Reactivity and Burnout The char burnout behaviour in pf combustion is thought to be related, not only to coal particle size, but also to the different char morphologies (and hence rank) generated during the early stages of combustion. These factors have a direct impact on char reactivity and consequently, burnout. Char morphology is related to the temperature history experienced by the coal, as well as petrographic composition, volatile/tar yield during pyrolysis, rank, and ash content and distribution in the coal particle. These properties seem to influence the morphologies of the char particles which vary from solid particles with very low porosity (fusinoid-solid/inertinoid) to highly porous bubbles (tenuspheres and crassispheres), or intermediate mixed network chars. For a particle of caking coal exposed to a rapid heating rate, the core of the particle may become fluid due to mesophase formation, while a skin of resolidified material of variable thickness maintains the particle integrity. Figure 7(a) shows a representation of that process. Under such conditions, the swelling of the coal particle depends, not only on volatile matter, but also on the porosity of the outer layer. The generation of gases and vapours, and their heating, within the particle core increases the internal pressure of the particle resulting in an expansion of the solid boundary as shown in Figure 7(a). Conversely, the loss of gases and vapours through the pores tends to decrease the internal pressure. If the gas loss and expansion are not fast enough to compensate for the increase in internal pressure inside the particle then the outer layer will rupture with the ejection of fluid and gases over distances of much greater than the particle radius, or even particle fragmentation. This is particularly apparent for large particles where the temperature gradient through the particle results in a thick solid outer layer. For smaller diameter particles the temperature is expected to be much more uniform through the particle and the outer resolidified layer much thinner, or non-existent. Thus, the pressure increase within the particle can be accommodated easily by particle swelling and steady evaporation of gases/vapours. This model, which has been quantified by the authors, has been confirmed by high speed video recording of heating coal particles. Depending on the distribution of the macerals within the coal, different char cenospheres are produced as shown in Fig. 7(b). Coal heterogeneity may account for the different types of char morphologies observed for pf coal. Certainly, for the DTI-NOx coals are power station coals with a restricted range of rank, no obvious relationship has been determined between maceral content and evolved hydrocarbon gases during pyrolysis. These coals consist of about 8 0 % network chars, and 1 0 % each of the extreme cases. 8 |
