Numerical simulation of propane MILD combustion in a lab-scale cylindrical furnace

Abstract

This paper reports a CFD numerical study of propane (C3H8) combustion inside a laboratory-scale cylindrical furnace under moderate or intense low oxygen dilution (MILD) regime. The reliability of the present numerical simulation was firstly validated against previous experiment (Energy & Fuels, 2018, 32(8): 8817–8829), and the C3H8 MILD combustion characteristics were thoroughly examined through a comparison with its traditional combustion counterpart, in terms of temperature distribution, heat release behavior, CO formation, and NO formation as well as reduction mechanisms. The results show that, MILD combustion produces a lower overall heat flux on furnace walls than traditional combustion, while the proportion of convection heat transfer increases remarkably, suggesting the different heat transfer behaviors between the two combustion modes. In addition, both MILD and traditional combustion modes exhibit only the positive heat release region inside the furnace, however, C3H8 consumption pathway indicates that traditional combustion features strong C3H8 pyrolysis in the early ignition stage (x = 80 mm), which is not found in MILD combustion. Moreover, the formation of CO in traditional combustion is dominated by the oxidation of larger hydrocarbon molecule in the intense reaction region (x = 240 mm), while CO in MILD combustion comes from the oxidation of smaller hydrocarbon molecules in both early ignition and intense reaction regions, showing its volumetric reaction feature. Furthermore, the relative contributions of the prompt route as well as the N2O and NNH intermediate routes to final NO emission are enhanced in MILD combustion comparing to those in traditional combustion, however, these routes still can be neglected in both traditional and MILD combustion due to their significantly low yields. For NO reduction, reburning route plays a more important role than SNCR route in both traditional and MILD combustion, and is even more profound in the latter. Overall, thermal-route still dominates the NO formation for both combustion modes in the present furnace.