OCR Text |
Show The present state of the art of beneficiation limits ash reduction to about 4 percent for some coals and does not take into consideration the heterogeneous nature of the coal ash. Although the total ash is reduced, there is no guarantee the quality of the washed product has been improved with regard to ash chemistry. Micronizing of coal to 3 ym average particle size can be achieved at the expense of a large power consumption at the mill. Although experimentation has demonstrated a substantial reduction in flame length, local furnace slagging because of increased heat release may become a problem. There is a good deal of uncertainty about how agglomeration within the combustion process may mitigate the advantages of superfine coal grinding. COMs simplify coal-handling problems when retrofitting existing oil-fired steam generators. The coal ash chemistry is the dominant factor in selecting a COM suitable for firing in steam generators. High-vanadium oils (i.e., >200 ppm) may cause slagging problems with coal rich in iron if the quantity of coal mixed with the oil is small. Combustion problems may also be a source of fireside deposits in the upper reaches of the furnace, particularly in porous coals where the burnout time appears to be longer than for the raw coal. Prediction of slagging and fouling from the ash chemistry of coal is still very much an art rather than a science. Because of the heterogeneity of the fuel and the variability of the designs, each candidate for retrofitting with direct firing of pulverized coal must be evaluated as a special case. 1-62 |