The future of pulverized-coal firing in Great Britain

The success attained in the burning of semi-bituminous and bituminous fuels has established pulverized-fuel firing in dry-ash bottom furnaces as the preferred method of combustion for power stations in Great Britain. The tendency to divert a high proportion of the poorest quality coal to these new power stations should be viewed with alarm. Owing to the variable quality of ash and the limitations imposed by the presence of this refuse in increasing quantity, boiler units must be moderately rated to maintain satisfactory availability. Combustion rates must be controlled to obviate slagging and birdnesting with the most unfavourable fuel, and this involves the provision of comparatively large furnaces for the duty. For boiler units much in excess of 400 000 lb/h a strong case can be established for using specially-selected fuel, or, alternatively, liquid ash-removal may be employed. In either case, the fine dust from the arresters must be disposed of, and this frequently proves both difficult and expensive. With a view to solving this serious problem, as well as offsetting the disadvantages of variable ash characteristics, the cyclone furnace has been evolved and is at present undergoing trials in the United States, whilst overfeed firing with moving grates has already been developed there for application to boiler units of suitable size for power-station work. Some considerable increase in the output of dust fuel appears probable as the result of increased mechanization in the mines, so that in these circumstances pulverized-coal-fired boilers will always be in demand to dispose of this, unless it is eventually found more profitable to upgrade it for other processes.