Book - School of Science and Technology

298 Fuels, storage and handlingvehicle. The latter will be equipped with either a belt conveyor or with facilities forpneumatic discharge. It should be the aim of delivery methods to avoid degradationand segregation of the fuel, i.e. not to break up the larger pieces and not to separateout the large from the small within the mix.Delivery by tipper vehicle is more appropriate to larger plants, rated perhaps at above8 MW although there can be no absolute rule as to minimum rating, and to those whichare sited away from occupied buildings. Tipping might be through a road grid into anunderground store or into a captive tippler hopper as described later. For a smaller plantor one sited near to areas sensitive to noise and dust, totally enclosed pneumatic handlingis particularly suitable.StorageSolid fuel may, of course, be stored in the open without cover and for very large plants inindustry this may be the method adopted. In other circumstances, the site and arrangementsselected for storage may very well depend upon the economics of excavation forunderground bunkers, the practicability of arranging overground silos, the necessity formechanised fuel handling and the method of boiler firing to be adopted. An example ofthe last influence is the use of automatic underfeed stokers, particularly where these are tobe of bunker-to-boiler type (see p. 281).It is necessary in designing all types of silo or bunker to take into account the naturalangle of repose which the fuel will adopt when heaped and the consequent necessity toslope the sides of the store to encourage movement, both as shown in Table 11.3. Whensilos or bunkers are constructed on a permanent basis in concrete, either underground orat surface level, it is wise to provide flexibility for a possible future change in use (to houseoil tanks for example) by arranging for the structural walls to be vertical and forming theinternal sloping faces in falsework.An overground silo may be built in any of a number of materials and in some cases maybe only a semi-permanent structure built up from pre-fabricated sections. Ferrous sheetsof cast iron or mild steel, unless provided with very substantial protection, are not asuitable material for use in construction of storage for a wet and acidic material such ascoal.Table 11.3 Properties of coal (physical data)Angle to horizontal (deg:)Size range (mm)Hopper sidesGroup nameBulk density a(kg/m 3 )Upper Lower In repose Steel ConcreteTrebles 90±64 50±40Doubles 580±700 57±45 40±25 40 40 47Singles 40±25 25±13Peas 20±13 13±6Grains 700±800 11±6 6±3 55 55 60Smalls b 50±25 Nonea Dry coal loosely packed. b Smalls are not a graded group.