Evaluation of gas radiation modeling in oxy-fired furnaces
Paper in proceedings, 2010
Oxy-fuel fired furnaces will exhibit combustion conditions different from air-firing and this will have important effects on the radiative properties of the gas. In contrast to nitrogen (N2), carbon dioxide (CO 2) and water vapor (H2O) are strong infrared emitters and the radiative activity of the gas in oxy-fuel combustion is increased compared to air-firing. In an oxy-fired furnace the pressure path-lengths are several times larger than in an air-fired furnace and the ratio of H2O to CO2 can be significantly different. These differences between air and oxy-fuel combustion make the role of radiative heat transfer by gases in oxy-fired combustion different to air-firing, even in cases where the temperature distribution is similar to that of airfired combustion. These aspects are essential for design of oxy-fired furnaces and need to be accounted for when temperature and heat transfer conditions are determined by modeling. In comprehensive combustion models, for example CFD models, it is common to neglect the spectral variations of the gases and to treat the spectrum by a single average, i.e. a gray approximation. Approximate models frequently applied in combustion modeling, such as the Weighted-Sum-of-Gray-Gases (WSGG) model, are not suitable for oxy-fired boilers since their parameters are fitted to pressure path-lengths, and ratios of H2O to CO2, typical for airfired conditions. Different approximations used to account for gaseous radiation in CFD-simulations are investigated: temperature predictions by a gray model and a non-gray formulation of a WSGG model are compared in air- and oxy-fired conditions. The WSGG model used in this work is suitable for oxy-fuel conditions since it accounts for various ratios of H2O to CO 2 and the parameters are fitted to a broad range of pressure path-lengths. The modeled case is a propane flame in Chalmers 100 kW oxy-fuel rig. Two flames with similar temperature distribution are modeled and compared: an oxy-fuel case with 27 vol.% oxygen in the feed gas and an airfired reference case. To support the CFD-results, calculations of the radiative source term are carried out for a domain between two infinite plates with similar temperature and concentration profiles as in the CFD-simulations. In these calculations, the gray model and the non-gray formulation of the WSGG model are compared with a Statistical-Narrow-Band model. In addition, the role of radiative heat transfer is investigated by including soot radiation in the CFD-simulations to give an understanding of the relative importance of gaseous radiation when soot particles are present. It is shown that a use of a non-gray approach is motivated when modeling oxy-fuel combustion since the gray model fails in predicting the source term, which affects the predicted temperature field in CFD calculations. Furthermore, the inclusion of soot radiation is more crucial for the modeled flames than the use of a more rigorous description of the radiative properties of the gaseous components.