ABSTRACT
This paper presents experimental and modelling work on the flame characteristics of oxy-fuel combustion. The paper focuses on the radiative heat transfer and discusses the difference in emissivity between oxy-fuel and air-fired environments. The experimental data presented concern in-furnace temperature and uni-directional total radiation profiles measured in a 100 kWth test unit which facilitates oxy-fuel combustion with real flue gas recycle. The test unit is capable of firing both gas and coal. This paper deals with gas-firing where the tests comprise a reference test in air and two oxy-fuel test cases with different recycled feed gas mixture concentrations of O₂ (21 and 27 vol %) and CO₂ (79 and 73vol %). The 21 vol % case (termed OF 21) was chosen to keep the volumetric flow conditions the same as for the reference tests in air and the 27 vol % case (termed OF 27) was chosen to attain similar combustion temperatures as for air. The total mean emissivity of the air and oxy-fuel flames is determined by using the Schmidt method and the gas emissivity is discussed by means of an available model, suitable for gas emissivity predictions of the oxy-fuel environment.
The results clearly show that the combustion temperatures are decreased drastically for the OF 21 case compared to air-fired conditions. When going from OF 21 to OF 27 conditions the amount of recycled flue gas is reduced and the in-flame temperatures approach those during air-fired conditions, but with a significant increase in the flame radiation intensity in the high temperature zone. Despite the lower temperature levels in the OF 21 case compared to the air case the radiation intensity levels approach those of the air case. This shows that the emissivity of the two oxy-fuel environments is clearly different from the air-fired atmosphere.