Experimental and Numerical Study on Moderate or Intense Low-Oxygen Dilution Oxy-Combustion of Methane in a Laboratory-Scale Furnace under N₂, CO₂, and H₂O Dilutions

Abstract

This paper reports an experimental and numerical study on the moderate or intense low-oxygen dilution (MILD) oxy-combustion characteristics of burning methane in a non-premixed cylindrical furnace. Specifically, the present work aims to investigate the effects of different diluents (N2, CO2, and H2O) and oxygen concentrations in the oxidant (or the dilution level) on flame stability, combustion, emission, and radiation heat transfer characteristics. Results show that CO2 and H2O dilutions, even for a high oxygen concentration of 30%, are more beneficial than N2 dilution for realizing the flameless MILD oxy-combustion. Specially, the CO2 dilution results in the most uniform in-furnace temperature distribution and the lowest peak temperature, as well as the most uniform radiation heat transfer on the furnace wall. Moreover, the H2O dilution leads to the lowest CO and NO emissions and the largest proportion of radiation in heat transfer on the furnace wall. Nevertheless, although N2 is superior to CO2 and H2O as a diluent for establishing stable MILD combustion at low oxygen levels, the oxygen-enriched MILD-N2 combustion exhibits quite high NOx emissions. In addition, it is interestingly found that the previous criteria of Cavaliere and de Joannon (Prog. Energy Combust. Sci. 2004, 30, 329–366) and Wünning and Wünning (Prog. Energy Combust. Sci. 1997, 23, 81–94) for flameless MILD combustion cannot apply under the N2 dilution but still work under CO2 and H2O dilutions for high oxygen concentrations.