An experimental study on MILD combustion of prevaporised liquid fuels

• MILD combustion of prevaporised liquid fuels under elevated pressure is examined.
• MILD combustion is achieved for all fuels under atmospheric pressure.
• Combustion stability is highly dependent on fuel type.
• Pressure, jet velocity and carrier gas show great impacts on NOx emissions.

This paper presents an experimental study on moderate or intense low oxygen dilution (MILD) combustion of prevaporised liquid fuels burning in a reverse-flow MILD combustor under elevated pressures. The influence of fuel type, equivalence ratio, carrier gas, operating pressure and air jet velocity on the combustion stability and emissions are investigated. Ethanol, acetone and n-heptane are vaporised and carried to the combustor using either nitrogen or air. It is found that the combustion stability is highly dependent on fuel type, with n-heptane being the most unstable due to its fast ignition under all high-pressure conditions studied. Measured CO emissions emitted from all fuels are very low except when the equivalence ratio approaches the lean extinction limit, and this effect is not dependent on the pressure. The joint regime of low CO and NOx emission becomes narrower under elevated pressure as NOx emissions emitted from all fuels increased with pressure. The enhanced NOx formation rate via the nitrous oxide mechanism, the slower mixing, the increased flame temperature and residence time are believed to cause higher NOx emissions as pressure increases. The NOx emissions are reduced by increasing the air jet velocity, which is attributed to a lower peak temperature. The NOx emissions are also reduced when the fuel is carried by nitrogen instead of air. Further research is required to understand this trend which will help in reducing NOx emissions under these conditions, especially at elevated pressures.