Coal particle fragmentation

Coal particle fragmentation and attrition during combustion has been extensively studied. For bubbling fluidized bed combustion, the attrition rate has been typically assumed proportional to the excess gas velocity above minimum fluidizing conditions, carbon... 

Fifiwara, N., Inokuchi, K., Matsuoka, S and Yamada, T. A Study on Fragmentation of Coal Particles in a Circulating Fluidized Bed Combustor, in Circulating Fluidized Bed Technology III (Basu, P., Horio, M., and Hasatani, M., eds.), pp. 373-378. Pergamon Press, Oxford (1991). 

Applications include coal and trash combustion, coal and biomass gasification, and chemical, including polymer, synthesis. Both particles and the fluidizing gas stream change properties during the process. Coal is converted to ash, carbon dioxide, and carbon monoxide. The coal particle can also become fragmented. Coal and steam can yield hydrogen and carbon monoxide. 

Numerical modelling of outbursts requires the quantification of a number of processes and factors. These include gas adsorption and desorption, coal deformation and failure, coal fragmentation, gas dynamics and transport of outburst coal (particle flow). 

The rates of the fragmentation reactions in the coal feedstock are rapid and, for the most part, nonreversible (unless combination with other moieties to form coke occurs) and depend on the nature of coals. The microcomponents, as identified by the microscopic examination of coal particles as vitrinite and exinite, are the most reactive materials that undergo rapid fragmentation. The other components—semifusinite and fusinite— produce insignificant amounts of liquid products but may act as nuclei for coke formation, which might be deleterious to ultimate conversion. 

The thermal decomposition process, while written as an orderly process, may be quite disordered. Nevertheless, it is proposed that as the coal particle temperature rises during thermal decomposition (which may also be the initial stages of the combustion process) (Chapters 14 and 15), the bonds between the aromatic clusters in the coal macromolecule break and create lower-molecular-weight fragments that are detached from the macromolecule—the larger fragments of this decomposition process are often (collectively) referred to as the metaplast. 

In the first stage of coal liquefaction, finely-crushed coal (particle size <0.1 mm) is slurried with a solvent, to render the coal flowable and pumpable. The choice of solvent is particularly important, since it must be suitable to stabilize the coal fragments and to dissolve the smaller disintegrated molecular moieties. Due to their similarity in chemical nature, coal-derived oils are particularly efficient. Anthracene oil from coal-tar processing was therefore preferred as a solvent, when coal hydrogenation was being developed. 

His measurements were made on "bulk" coal fragments extracted from the vicinity of pyrite particles. Electron-optical methods, though, can make such measurements in situ and have better spatial resolution than his bulk methods allowed. 

The CaO from CaCOs decomposition, the other new specie involved in the combustion mechanisms at LCL runs, is a solid porous material, which behaves like a fluid at the fluidised bed. That means that its interactions with radicals from coal are not limited and the radicals could be adsorbed [9] into its porous structure, hindering their total oxidation and, in consequence, promoting their interaction. This fact is corroborated by the Coronene formation in LCL experiments. Coronene (Co) is the most stable of the PAH studied and the radicals trend in their stabilization will be towards Co formation. Besides, as result of the fluidisation movement and the high temperature, CaO can be fragmented into smaller particles, elutriation and attrition phenomena, the smaller particles formed undergoing entrainment by the airflow. 

Information on the velocity of the particle is found by measuring the transit time of the particle through the sample volume [104]. The instrument has been used in large scale pulverized coal boilers [105-108] char fragmentation and fly ash formation during pulverized coal combustion [109] and for coal slurries [110,111]. 

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