Flame and radiation characteristics of gas-fired O2/CO2 combustion

This paper presents an experimental study on the flame properties of O2/CO2 combustion (oxy-fuel combustion) with focus on the radiation characteristics and the burn-out behaviour. The experiments were carried out in a 100 kWth test unit which facilitates O2/CO2 combustion with real flue gas recycle. The tests comprise a reference test in air and two O2/CO2 test cases with different recycled feed gas mixture concentrations of O2 (OF 21 @ 21 vol.% O2, 79 vol.% CO2 and OF 27 @ 27 vol.% O2, 73 vol.% CO2). In-furnace gas concentration, temperature and total radiation (uni-directional) profiles are presented and discussed. The results show that the fuel burn-out is delayed for the OF 21 case compared to air-fired conditions as a consequence of reduced temperature levels. Instead, the OF 27 case results in more similar combustion behaviour compared to the reference conditions in terms of in-flame temperature and gas concentration levels, but with significantly increased flame radiation intensity. The information obtained from the radiation and temperature profiles show that the flame emissivity for the OF 21 and OF 27 cases both differ from air-fired conditions. The total emissivity and the gas emissivity of the OF 27 and the air-fired environment are discussed by means of an available model. The gas emissivity model shows that the increase in radiation intensity (up to 30%) of the OF 27 flame compared to the air flame can partly, but not solely, be explained by an increased gas emissivity. Hence, the results show that the OF 27 flame yields a higher radiative contribution from in-flame soot compared to the air-fired flame in addition to the known contribution from the elevated CO2 partial pressure.