OCR Text |
Show but it is inappropriate as an alternative to stoker firing of washed coals, because of the cost of coal crushing and the high fan power associated with the deep bed. When the NCB recognised in 1972 that coal would again become economic as a fuel for industry, a new development programme was initiated to (3) adapt fbc to make it more suited to industrial requirements . Preliminary tests showed that the standard commercial grades of washed coal, singles and smalls, could be fired directly to the bed without any on-site preparation. Furthermore, the combustion performance was improved in comparison with crushed coals and it was possible to reduce the bed depth to about 0.15 m static, i.e. about 0.3 m fluidised. An acceptable combustion efficiency was achieved with singles coal within a chamber height of 1 m, but a taller chamber and grit refiring was necessary with smalls. These tests were followed by the operation of a small prototype boiler at the Coal Research Establishment, commissioned in 1974. The expertise and performance data from this boiler and associated test unit were used to design four demonstration prototype boilers, installed at field trial sites. Three of these were shell boilers , which were designed in 1976, commissioned in 1977/78 and are now fully meeting site heating requirements. The fourth was a 30 MW water tube boiler, commissioned in 1981. These prototypes have led to commercial designs of fbc boilers in collaboration with several manufacturers, and a number of these units are already in use at industrial sites. Other manufacturers, recognising the advantage of fbc demonstrated by the NCB, initiated independent development programmes and are also offering fbc boilers on a commercial basis. A vertical shell boiler offers the most appropriate design for fbc for outputs up to about 5 MW (Figure 2). Two of the NCB prototypes were of this type, both rated at 2.5 MW, one for steam and the other for hot water. The success of these led to the commercial designs of EMS Thermplant Ltd, Wallsend Slipway Engineers Ltd and Allied Boilers Ltd. Eight vertical fbc boilers are now in use at industrial and horticultural sites, providing a high level of amenity and reliability, and further orders are expected. 20-11 |