MILD oxy-combustion of gaseous fuels in a laboratory-scale furnace

Abstract

The present study investigates the characteristics of Moderate or Intense Low-oxygen Dilution (MILD) oxy-combustion in a laboratory-scale furnace. Experiments using natural gas (NG), liquefied petroleum gas (LPG) and ethylene (C₂H₄) are carried out at a firing rate of 13 kW. The furnace temperatures and exhaust emissions are measured for a range of equivalence ratios and external-CO₂ dilution rates. It is observed that MILD combustions occur for the three fuels even when using pure oxygen as oxidant. When diluting oxidant by CO₂ at a fixed rate, the MILD combustion can be established as long as the equivalence ratio (Φ) is sufficiently high. The region of MILD combustion is found to be wider with dilution by CO₂ than by N₂. Notably, also, the operating range of MILD combustion is larger for NG than LPG or C₂H₄ as fuel. Moreover, when Φ < 1, as Φ is increased, the furnace temperature rises slightly but the NO(x) emission decreases. This cannot be explained when using the traditional thermal NO(x) mechanism. Indeed, using various NO mechanism models, our calculations show very low NO emissions resulting from the thermal, prompt and NNH routes but a much higher value from the N₂O-intermediate route. Namely, only the latter mechanism plays a crucial role in forming NO. Also important is that the NO reburning appears to reduce NO emissions notably and so should not be ignored in the MILD combustion.