THE PERFORMANCE OF PULVERIZED-COAL FLAMES IN A SIMULATED COMBINED-CYCLE UNIT

Measurements have been taken in a pulverized coal-fired furnace to eva luate the combustion characteristics of a combined cycle unit--a coal- fired boiler coupled with a gas/oil-fired turbine. Results are present ed for two aerodynamically distinct laboratory scale burners-the singl e annular orifice and the single central orifice (SAG and SCO) in a 0. 5 MW furnace. The combined cycle is simulated through the vitiation of combustion air delivered to the burners. Under unvitiated-air conditi ons, both laboratory burners offer similar combustion performance in r egard to particle burnout. The SAO burner, representative of wall-fire d practice, exhibits much higher NOx emissions due to the almost immed iate evolution of the fuel's volatiles in an oxygen-rich shear zone im mediately downstream of the burner. Its counterpart (SCO burner) gives less NOx due to a manipulation of the aerodynamics, with the volatile s being liberated in the oxygen-lean internal recirculation-zone. Viti ated-air conditions produced by increasing the flue-gas recirculation, altered the burner's performance. The NOx reduction in the faster mix ing burner is substantial, from 667 to 171 ppm, with some reduction se en in burnout. For the slower mixing burner, with aerodynamics more ap propriate to staged combustion, low-NOx operation is observed with a r eduction from 356 to 250 ppm. More importantly, the stability limits o f this burner are increasingly restricted with air vitiation. A mathem atical model of coal combustion was also used to predict the observed trends of flame stability and to establish a single criterion for flam e stability on both the fast and slow mixing burners operating under n onvitiated, as well as vitiated-air conditions. Flame stability strong ly depends on combustion aerodynamics in the burner, while, under viti ated-air conditions, the local concentration of oxygen becomes a domin ant factor. For all the operating conditions simulated by the mathemat ical model, it was found that the appropriate criterion for defining a stable flame is that the axial temperature profile reaches its peak v alue (more than 900 degrees C) within a distance of approximately 10 b urner diameters. This is the condition under which volatiles are well released from the coal and burnt in the heat-confined environment to s ustain a high temperature. The model overpredicts the stability limit; this indicates a limitation of the kappa - epsilon model for flow wit h a high swirl number. It is suggested that the slow mixing of fuel an d oxidant associated with low-NOx burner technology in a combined cycl e operation would prevent it from achieving its expected capabilities. The results of this study suggest that a burner offering less stratif ication of air and fuel would be more appropriate. (C) 1997 by The Com bustion Institute.