EXPERIMENTAL INVESTIGATION OF NOx FORMATION MECHANISMS IN A FUEL-OIL BOILER

The work whose results are reported in this paper was aimed at gathering information that can
give a deeper insight into the structure of a diffusion fuel-oil flame. This allows us to get a
better understanding of the combustion process and thus a correct basis for combustion
modeling, focused on NOx formation. In order to attain the objective, axial and radial
temperature, NOx and O2 concentration profiles have been measured in the flame for high and
low firing rates of a domestic hot water boiler (around 424 and 270 kW respectively).
Temperature was measured by means of two B-type thermocouples (diameters 0.5 and 0.35
mm) and NOx and O2 concentrations using a classical gas analyzer. Maximum NOx emissions
(167 ppm at 424 kW and 134 ppm at 270 kW) were recorded in high oxygen concentration
and low temperature zones of the flame ( ≈ 1000°C), where no thermal-NO can be formed.
Flue gas recirculation and NOx destruction process is clearly demonstrated in the flame due to
the higher NOx concentrations recorded in the combustion chamber near the burner exit than
in the chimney. Near the burner exit NOx, O2 and temperature radial profiles are very
unstable. Higher temperature fluctuations were detected with the small thermocouple in the
measurements at 270 kW. The contribution of the three types of NOx formation mechanisms
was obtained using a simplified model. According to these results, the majority of NOx in this
type of flame could be caused by the prompt-NO mechanism, formed in the zone close to the
burner exit at the location where secondary combustion air is introduced.