Book - School of Science and Technology

300 Fuels, storage and handlingReserve fuel storeSmall storagebunkersAsh silo andconditionerAuto-tipperClinker crushersStokersFigure 11.1 Typical composite handling plant for coal and ashLiquid fuelOf the two principal types of liquid fuel, petroleum oil and the coal tar series, it is theformer only which need concern us here. Crude petroleum oil, a complex mixture ofparaffins, aromatics and naphthalenes, is subjected at the refinery to heat treatment andother processes leading to progressive decomposition. The various `families' of componentsare segregated and subsequently re-grouped into commercially usable products,bitumen being the ultimate residual. It will be appreciated, therefore, that the five gradesof petroleum oil which are now available, as listed in Table 11.4, are not unique compoundsbut products created by selective blending. Rising or falling demand for a givenproduct in the chain may upset the viability of the whole and lead to a quite significantreappraisal of unit costs overall.At one extreme, kerosene (grade C2) is rarely fired to other than small domestic boilersrated at not more than about 50 kW. Of the remainder, the heavier the grade, the less theunit volume cost but the greater, and more costly, the facilities required for storage,handling and burning. A total economic judgement has to be undertaken, taking all theseaspects into account before a selection can be made. For instance, light fuel oil (grade E),was for 20 years or more a much used technical/economic compromise, but can no longerbe so classified and thus is now little used.As far as the various characteristics of the oils listed in Table 11.4 are concerned, thefollowing should be noted:Flash point (closed). This is the minimum temperature at which, when the oil is heated, aflash may be obtained within the apparatus. Whilst the flash point is a limiting factor