ABSTRACT
The differences in soot-related radiation intensity between two different oxy-fuel flames and an air-fired flame was investigated in a 100 kW oxy-fuel test unit firing propane. The oxy-fuel test cases with 21 and 27 vol % O₂ in the recycled flue gas (RFG) were run with different amounts of dry recycled flue gas, which in principle only consisted of CO₂ from combustion and some excess O₂. The stoichiometric oxygen-to-fuel ratio was kept at 1.15 in all cases. Total radiation intensity was measured with a narrow angle radiometer. Temperature and gas composition measurements served as input to computations of gas radiation. A comparison of the computed gas radiation with the measured total radiation intensity enables to estimate the radiation related to soot. Clear differences were observed in the amount of soot formed between the two oxy-fuel flames (and also compared to the air flame). In the oxy-fuel flame with 21 vol % O₂ in the RFG, the soot formation is almost completely suppressed, but when the total flow through the burner is reduced with about 20% (by volume), i.e., from 21 to 27 vol % O₂ in the RFG, the amount of soot present in the flame becomes significant. This difference in soot volume fraction affects the radiation emitted from the flames; images of the flames qualitatively confirm these differences in the flame luminosity. Thus, carbon dioxide not only increases the gas radiation, but it can also drastically change the radiation originating from soot during oxy-fuel combustion.