Book - School of Science and Technology

Boiler firing ± solid fuel 281Coking (Figure 10.16(c))The operating principle in this equipment rests with the action of a reciprocating rammechanism which pushes fuel forward from an overhead hopper at the boiler front anddeposits it on a coking plate in the hottest area of the furnace. At that point, the volatilesare consumed and the residual coke is then passed towards the boiler rear by the action ofa moving grate. Early designs used a deep fuel bed and were, in consequence, unsuited toany type of shell boiler: this problem was later overcome when `low-ram' patternsappeared. Singles and doubles are the appropriate sizes of a coal having slight cakingproperties and an ash content of not more than about 8% (coal Rank 700±900).Moving bed (Figure 10.16(d))A chain grate stoker, as the name suggests, has a fire grate made up of a continuous chainof links or bars which run over driving sprockets at the boiler front. Fuel is fed to the grateby gravity from a hopper, the depth of the bed being controlled by a guillotine platemounted above the furnace entry. The position of an `ignition arch' behind the guillotine isarranged to expose it to maximum radiation so that it may raise and maintain thetemperature of incoming fuel. Combustion takes place progressively, downwards fromthe ignited upper layer of fuel towards that in contact with the grate. Wet smalls is thepreferred fuel (coal Rank 700±900).Hopper or bunker underfeed stokersIn the simplest form of such equipment, fuel is fed from a hopper down to a worm orscrew at its base: this is rotated at slow speed through reduction gearing from a motordrive. Beyond the feed point the worm is enclosed within a tube and serves to convey thefuel into a fire-pot which is built into a refractory base inside the boiler, as shown inFigure 10.18. Safeguards are provided, in the form of shear-pins or a slipping clutch, toprevent damage to the worm if jammed by a foreign body or by oversize coal.A second tube delivers air to the fire-pot from a fan driven by the same motor thatpowers the worm. This air is discharged through a series of slots or openings, disposedaround the fire-pot to form tuyeÁres which will not be fouled by fuel or ash. Secondary airis supplied by the same fan and preheated in order to control smoke emission.ClinkerTuyeresAir supplyboxWorm feedFigure 10.18 Fire-pot and fuel bed for underfeed stoker