THEORETICAL AND NUMERICAL INVESTIGATION ON FLAMELESS COMBUSTION

Abstract

Flameless combustion has been acknowledged as one of the most interesting combustion technologies to meet both the targets of high process efficiency and low pollutant emissions. A theoretical and numerical investigation, carried out to provide a consistent physico-chemical explanation of this combustion regime, is reported here. CFD simulations of the flameless combustion WS-FLOXTM burner have been performed at ENEA to study its thermo-fluid dynamic features; detailed chemical kinetics calculations, by means of a specific zero-dimensional loop reactor model, have been performed (still in ENEA) to analyze its chemical aspects. Process temperature turns out to be the controlling factor in combustion dynamics: lean premixed conditions at lower temperatures, volumetric “flameless” combustion at higher tempera tures. This latter regime is characterized by strong interaction between kinetics and turbulence (Da ≅ 1): thermo-chemical fluctuations are respon sible for reaction zone “derealization” and nonequilibrium effects. Radiative losses from the reaction zone are larger than in conventional flames because of the larger energy absorption by CO2 and H2O present in recirculation gases: They contribute significantly to reduce combustion temperature. The flame-less combustion regime is explained by recirculating gases impact on the reaction zone, i.e., a mass effect (dilution lowers overall combustion tem perature), a thermal effect (enthalpy addition to reactants promotes auto-ignition), and a kinetic effect as well (water and radicals affect autoignition mechanism and NOx formation). Sensitivity analysis has been performed to point out the most important NO,v formation mechanisms. The enhanced action of water removal reactions (with respect to conventional combustion) has been found to provide a realistic kinetic explanation for the high NOx emissions reduction, complementing the well-known thermal explanation. CFD postprocessing calculations have also pointed out the high NOx sensi tivity to thermo-chemical turbulent fluctuations.