A Layer-over-layer Radiative Model for Predicting Flue Gas Emissivity in Oxygen-Enriched Combustion

Oxygen-enriched combustion along with fast-responded intelligent control has a key role to play in clean operation of modern industrial furnaces. However, there is lack of a model for estimating flue gas radiation emissivity, which give a right balance between computational efficiency and prediction accuracy. Present radiation models such as line-by-line (LBL) model have a high accuracy but with an unacceptable computation cost, while weighted-sum-of-gray-gases (WSGG) model is the reverse. Herein, a computationally efficient Layer-over-layer model (LOL) without sacrificing accuracy is developed for radiative energy calculation based on "spatial accumulation after layered spectral modeling". Through modeling at temperatures of 600–2000 K, pressures of 0.1–65.0 bar, path-lengths of 0.1–20.0 m, and H2O(g)/CO2 ratio of 2:1, the LOL model was found to be as accurate as the LBL model with a maximum error below 2.0%, and computationally efficient as the WSGG model with some four orders of magnitude quicker than the LBL. The flue gas emissivity was found to be logarithmically dependent on radiative molecule number, total pressure (P) and the H2O(g)/(H2O(g) + CO2) ratio, while following a Gompertz-type dependence on temperature (T). An empirical formula for emissivity calculation was derived using nonlinear least squares regression, linking with the radiative molecule number, pressure, composition, and temperature. The empirical formula is capable of estimating chamber height for typical re-heating furnaces with oxy-enriched combustion. With 100% oxygen concentration at P = 1 bar and T = 1600 K, the H2O/CO2 ratio in flue gas is 2:1, and the flue gas layer thickness (a key parameter related to furnace height) can be reduced by 76.4% compared to that with conventional air-fueled combustion.