| OCR Text |
Show An example of a dilute phase system is a suction nozzle, shown in Figure 8, recently developed by the NCB for singles. The nozzle consists of two vertically mounted concentric tubes with the top of the inner tube connected to the conveying pipe and the outer tube forming an annulus. A suction fan draws air through the conveying pipe, inner pipe and annulus, entraining coal at the end of the nozzle. A cyclone above the boiler feed hopper is used to separate the coal from the air, prior to the fan. Several boilers can be supplied by incorporating diverting TY' valves in the pipework. This conveying system has the advantage that, apart from the fan, there are no moving parts. 3.4 Feeding systems Feeding equipment is required to supply the combustor with coal at the rate demanded by the control system to maintain boiler steam pressure or water temperature. With stoker firing, the feed system forms an integral part of the stoker. A screw conveyor is used for underfeed stokers and rotary valve or ram for coking stokers. Chaingrate stokers are self feeding by means of the variable-speed grate. Screw feeders or rotary valves are used to feed singles to fbc boilers and furnaces, but can become blocked with the most difficult smalls. One alternative is to use a short drag-link conveyor with variable speed drive, as used for feeding large spreader stokers. 3.5 Coal/liquid mixtures In recent years there has been considerable interest in the USA and elsewhere in producing a 'liquid' fuel by pulverising coal and mixing it with oil or water, together with additives to prevent settling out. The incentive is to provide a means of firing coal in existing boilers, designed for oil or gas, utilising the oil tanks for storage. Inevitably there would have to be some boiler down-rating because erosion by ash particles is likely at the high gas velocities used in boilers for clean fuels. Also, there would be modifications to the fuel handling system and burners, to accommodate particulates, and a fly-ash collector would be required to meet dust emission requirements. 20-25 |
