Experimental studies on heat transfer of oxy-coal combustion in a large-scale laboratory furnace

Abstract

A number of issues arise in the transition from air-firing to oxy-firing. In situations where the furnace should be retrofitted for oxy-firing, a similar heat transfer performance is crucial for operation of power plants. A survey of recent literature turned up very little detailed research on the characterization of pulverized oxy-coal flames generated by staged feed-gas burners and the impacts of burner settings on heat transfer performance. A large number of studies applied either O2 fraction upstream of the burner or recycle ratio as main parameter to characterize oxy-fired conditions. For this reason, this study applies other characteristic parameters, as for example swirl number and feed gas flow ratio between the burner registers, rather than only O2 concentration for operation of the test facility. Experiments were conducted in a test facility with a rated capacity of 0.40 MWth fired by an industry-type burner. A theoretical study was also carried out to calculate combustion parameters, e.g., adiabatic flame temperature, in order to define burner settings and assess the experimental results. The burner was set to operate at three levels of secondary swirl number (1.15, 1.65, and 2.05), while part of the feed gas was divided between the secondary and tertiary registers using flow ratios between 0.40 and 2.00. For completeness, three levels of O2 fraction upstream of the burner (29, 31, and 33 vol% O2) were also tested under oxy-fired conditions. Measurements of peak flame temperature, absorbed heat flux at the water-cooled walls, and total radiative heat flux were performed and applied besides heat balance calculations to evaluate heat transfer performance. Experimental data indicated the feed gas distribution among the burner registers control the heat transfer in the furnace in parallel with the swirl strength of the secondary flow for both air-fired and oxy-fired conditions. Even though lower flame temperatures were obtained in oxy-firing, the results suggest the possibility of matching similar heat uptake in the furnace to that found under air-fired conditions. Experimental data also indicated the choice of an O2 fraction of 31 vol% upstream of the burner, which enables a very close adiabatic flame temperature as air-firing, is an appropriated parameter for maintaining similar heat transfer conditions in the furnace.