96/01905 Experimental and computational study of turbulent heat transfer characteristics in serrated channel flow

Abstract

Measurements have been taken in a pulverized coal-fired furnace to evaluate the combustion characteristics of a combined cycle unit—a coal-fired boiler coupled with a gas/oil-fired turbine. Results are presented for two aerodynamically distinct laboratory scale burners—the single annular orifice and the single central orifice (SAO 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 conditions, both laboratory burners offer similar combustion performance in regard to particle burnout. The SAO burner, representative of wall-fired practice, exhibits much higher NOx emissions due to the almost immediate evolution of the fuel's volatiles in an oxygen-rich shear zone immediately downstream of the burner. Its counterpart (SCO burner) gives less NOx due to a manipulation of the aerodynamics, with the volatiles being liberated in the oxygen-lean internal recirculation zone. Vitiated-air conditions produced by increasing the flue-gas recirculation, altered the burner's performance. The NOx reduction in the faster mixing burner is substantial, from 667 to 171 ppm, with some reduction seen in burnout. For the slower mixing burner, with aerodynamics more appropriate to staged combustion, low-NOx operation is observed with a reduction from 356 to 250 ppm. More importantly, the stability limits of this burner are increasingly restricted with air vitiation. A mathematical model of coal combustion was also used to predict the observed trends of flame stability and to establish a single criterion for flame stability on both the fast and slow mixing burners operating under nonvitiated, as well as vitiated-air conditions. Flame stability strongly depends on combustion aerodynamics in the burner, while, under vitiated-air conditions, the local concentration of oxygen becomes a dominant factor. For all the operating conditions simulated by the mathematical model, it was found that the appropriate criterion for defining a stable flame is that the axial temperature profile reaches its peak value (more than 900°C) within a distance of approximately 10 burner diameters. This is the condition under which volatiles are well released from the coal and burnt in the heat-confined environment to sustain a high temperature. The model overpredicts the stability limit; this indicates a limitation of the k − ε model for flow with a high swirl number. It is suggested that the slow mixing of fuel and oxidant associated with low-NOx burner technology in a combined cycle operation would prevent it from achieving its expected capabilities. The results of this study suggest that a burner offering less stratification of air and fuel would be more appropriate.