Improving the performance of fluidized bed boilers at Canadian forces base Summerside

Abstract

Canada's first coal-fired fluidized bed boilers were installed in 1982 at the heating plant for Canadian Forces Base Summerside in the Atlantic province of Prince Edward Island as a demonstration project jointly funded by Defence Canada and Energy, Mines and Resources Canada. Foster Wheeler Ltd. of St. Catharines, Ontario was the prime contractor with responsibility for supplying a turnkey heating plant including two boilers rated at 18 tph (40 000 lb/h) of steam each. The boilers subsequently met all contractual requirements pertaining to capacity, efficiency and emissions, but some problems emerged with respect to boiler tube erosion and other aspects of boiler operability. The paper describes four areas of difficulty and how they are being resolved. Evidence of boiler tube erosion appeared after only a few hundred hours of service and despite various preventative measures, resulted in a tube failure after 5600 h of operation. Subsequently studs were applied to the vertical membrane wall surfaces, and rods were welded longitudinally to the in-bed tubes. Since then, 4500 h of operation have been achieved, with no evidence of further tube wastage. A complex system for fly ash reinjection required an unacceptable level of maintenance, and in one instance failure of a minor component led indirectly to destruction of an induced draft fan. A much simpler, maintenance-free system was developed, based on the L-valve commonly used in circulating fluidized bed combustors. An inter-bed transfer gate, installed to permit one bed to be ignited from the adjacent one, was often inoperative due to warping. It was found that the same function could be achieved with a smaller, permanent opening. The capability of the boilers to fluidize cold material was limited, which led to difficulties in startup and frequent failure of the bed thermocouples. These problems were resolved by increasing the number of bubble caps in the distributor plate, and modifying the design of the thermocouple wells. The Summerside boilers are now considered to be reliable and acceptable in all major respects. It is concluded that there are no insurmountable barriers to bubbling bed fluidized bed combustion becoming a mature, dependable technology.