MILD combustion under different premixing patterns and characteristics of the reaction regime

Through experiment and numerical modeling, this study investigated the establishment of moderate or intense low-oxygen dilution (MILD) combustion in a laboratory-scale furnace when fuel and air are fully premixed (FP), partially premixed (PP), or non-premixed (NP). Experiments were carried out at firing rates from 7.5 to 15 kW and equivalence ratios (Φ) ranging from 0.5 to 1. The furnace thermal fields and exhaust NOx emissions for the three mixing patterns were compared. Validated computational fluid dynamics was used to aid in better understanding the flow and compositional structures in the furnace. Natural gas was used as the fuel. The eddy dissipation concept (EDC) model and the GRI-Mech 3.0 mechanism were used. Additional chemical kinetics calculations were also performed to examine reaction pathways under the MILD combustion regime. Moreover, the characteristics of the reaction regime of MILD combustion were examined and are discussed in detail. Estimation of the initial jet momentum rate (J) showed that JFP > JNP > J PP, and consistently the recirculating rate of internal flue gas (Kv) was found to be in the order Kv,FP > K v,NP > Kv,PP. Correspondingly, the highest values of both furnace temperature and NOx emission were experimentally measured in the PP case, while the lowest values were found in the FP case. The measured NOx emission was negligibly low for the FP case. Numerical results revealed that in all the three cases of firing natural gas (FP, PP, NP), more than 80% of the total NO formation results from the N2O intermediate route while other NO mechanisms are unimportant. As Φ is increased from 0.5 to 1.0, both the measured and simulated NO emissions in the three cases initially increase and then decrease. Moreover, for Φ > 0.9, the NO-reburning reaction becomes significant and the resulting reduction of NO is notable. The rates of both turbulent mixing and chemical reaction were found to play a significant role in the structure and establishment of MILD combustion, with estimated Damköhler numbers in the range Da = 0.01-5.35. © 2014 American Chemical Society.