Status of Topping Combustor Development for Second-Generation Fluidized Bed Combined Cycles

Addition of a fluidized bed combustor to a high-efficiency combined cycle plant enables direct firing of inexpensive run-of-the-mine coal in an environmentally acceptable manner. To attain high thermal efficiencies, coal pyrolysis is included. The low heating value fuel gas from the pyrolyzer is burned in a topping combustion system that boosts gas turbine inlet temperature to state of the art while the pyrolyzer-produced char is burned in the bed. The candidate topping combustor, the multi-annular swirl burner, based on a design by J. M. Bee´r, is presented and discussed. Design requirements differ from conventional gas turbine combustors. The use of hot, vitiated air for cooling and combustion, and the use of low heating value fuel containing ammonia, are two factors that make the design requirements unique. The multi-annular swirl burner contains rich-burn, quick-quench, and lean-burn zones formed aerodynamically rather than the physically separate volumes found in other rich-lean combustors. Although fuel is injected through a centrally located nozzle, the combustion air enters axially through a series of swirlers. Wall temperatures are controlled by relatively thick layers of air entering through the various swirler sections, which allows the combustor to be of all-metal construction rather than the ceramic often used in rich-lean concepts. This 12-in.-dia design utilizes some of the features of the previous 5-in. and 10-in. versions of the multi-annular swirl burner; test results from the previous projects were utilized in the formulation of the test for the present program. In the upcoming tests, vitiated air will be provided to simulate a pressurized fluidized bed effluent. Hot syngas seeded with ammonia will be used to simulate the low-Btu gas produced in the pyrolyzer.