Attrital coal

Attrital coal ground mass or matrix of banded coal in which vitrain and, commonly, fusain layers as well, are embedded or enclosed. 

Banded coal coal that is visibly heterogeneous in composition, being composed of layers of vitrain, attrital coal, and commonly, fusain. 

Lithotypes constituents of banded coal vitrain, fusain, clarain, durain, or attrital coal, or a specific mixture of two or more of these. 

Vitrinite maceral and maceral group composing all or almost all of the villain and like material occurring in attrital coal as the component of reflectance intermediate between those of exinite and inertinite. 

The modeling of fluidized beds remains a difficult problem since the usual assumptions made for the heat and mass transfer processes in coal combustion in stagnant air are no longer vaUd. Furthermore, the prediction of bubble behavior, generation, growth, coalescence, stabiUty, and interaction with heat exchange tubes, as well as attrition and elutriation of particles, are not well understood and much more research needs to be done. Good reviews on various aspects of fluidized-bed combustion appear in References 121 and 122 (Table 2). 

Chandler [Bull. Br Coal Util. Re.s. A.s.soc., 29(10), 333 (11), 371 (1965)] finds no good correlation of grindabihty measured on 11 coals with roh crushing and attrition, and so these methods should be used with caution. The Bond grindability method is described in the subsection Capacity and Power Consumption. ... 

Pulverization occurs by impact, attrition, or crushing. The capacity of a pulverizer depends on the grindability of the coal and the fineness desired, as shown by Fig. 27-19. Capacity can also be seriously reduced by excessive moisture in the coal, but it can be restored by increasing the temperature of the primary air. Figure 27-20 indicates the temperatures needed. For PC boilers, the coal size usually is 65 to... 

There can also be substantial particle attrition in cyclones in fluidized-bed systems because particles are accelerated at the inlet of the cyclone and impacted against the cyclone wall. Although there is little information on particle attrition in cyclones in the literature, it has been reported (Sishtla) that increasing system pressure decreases the attrition rate in cyclones operating with coal char. The mechanism by which this occurred was not determined. 

Arena et al. (1983) investigated the coal attrition in a mixture with sand under hot but inert conditions. As they increased the sand particle size while keeping its mass in the bed constant, they observed an increase in the coal attrition rate. They interpreted their results by assuming that the abrasion energy is shared out on the entire material surface. On the same basis Ray et al. (1987a) developed their attrition rate distribution model for abrasion in a fluidized bed. 

Figure 11. Variation of the steady-state attrition rate of coal ash (0.2 to 0.315 mm) with (Ug - Ugmj). (Pis et al., 1991.)...

The Antarctic coal beds are apparently less persistent, and locally may be thicker, than many of the beds in Paleozoic coal fields of North America. It is hazardous to generalize about petrographic composition from hand specimens that are available from many of the coal beds, but one obtains the impression that dull, moderately dull, and midlustrous attrital layers are more prevalent than in Paleozoic coal of the Northern Hemisphere. Vitrain bands tend to be relatively sparse and thin fusain chips and partings generally are present and may be abundant. Many coal specimens are relatively impure, apparently owing to well-dispersed detrital mineral matter. 

Figure 1. Electron micrograph of an ultrathin section of a dull attrital layer (durain) in a high volatile A bituminous (hvab) coal. V—vitrinitey E—exinite, M—granular micrinite. X 10,500...

The above calculations show a carbon loss of about 4 percent of the coal feed, primarily as fines produced by carbon attrition or by the shrinkage of the coal feed. As coal particle feed size increases the attritted carbon increases (note t in Eq. 21 is proportional to d ) but the elutriated carbon (Eq. 19) decreases. Carbon losses can therefore be minimized by the judicious choice of coal feed size. The simplified model presented above yields the following expression for the optimum size ... 

Kang, C. C. Johanson, E. S. Deactivation and Attrition of Co-Mo Catalyst during H-Coal Operation, in Liquid Fuels from Coal Academic Press, 1977 pp. 89-101. 

Catalysts for ebullating bed reactors are subjected to attrition. Those used for resid hydroprocessing and coal liquefaction must resist turbulence, from high gas velocities as well as erosion by the ash and impurities in the feedstocks. 

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. 

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