Development of a model vibrating-grate stoker for strongly caking coals

Research was undertaken to develop a stoker capable of satisfactory performance with strongly caking, low-ash-fusion coals, and adaptable to packaged boiler systems, for a capacity range of 50 to 1 000 lb of coal/h. To obtain a simple, compact design, a combination of an air-cooled vibrating grate, with a plate feeder extending the full width of the grate, was chosen. The feasibility of the design was demonstrated by experiments on a model stoker having a grate 1 ft wide and 2 ft long, housed in a refractory furnace. Grate surface temperatures of up to 200 F were recorded, and ash fused to the grate at average grate heat-release rates of about 225 000 Btu/ft 2 h. However, experiments on a full-size air-cooled vibrating-grate stoker installation showed that, with a water-cooled furnace and about 160 per cent total air, grate surface temperatures can be maintained below 500 F. Hence it should be possible to avoid the complication of water cooling the grate. A combination of experiments on the model stoker and full-sized vibrating-grate installations showed that a vibrating frequency of 1 200 c.p.m. is substantially more effective than frequencies of 400 to 800 c.p.m. in breaking up coke formations from strongly caking coal. It was established that the optimum fuel-bed thickness for good performance with strongly caking coal is 3 to 4 in, and that a positive means of providing a uniform fuel bed must be provided. Zone control of primary combustion air is also necessary. Data are given from tests on the model stoker with lignite, a non-caking coal, and a strongly caking coal. Extrapolation of the results to the design of full-scale stokers is attempted.