Experimental Investigation on Emissions and Stability of Combustion Processes with High Level of Internal Recirculation

Gas recirculation has been used for a relatively long time to stabilize combustion processes in several practical systems. To this aim several configuration have been considered based on fluid-dynamic stabilization by swirling or other strongly convoluted flow fields, as well as on external and internal gas recirculation or regenerative systems, trapped vortex [1-4]. It represents a challenging strategy to stabilize the oxidation process for novel combustion processes that aim at reducing pollutants emission, controlling the system working temperature by diluting the fresh incoming charge, and keep high process efficiency. The mass and sensible enthalpy ratio of recycled exhausted gas represents a key parameter to promote and stabilize the oxidation process. Of course, this issue is directly related to the main requirements that a combustion process has to satisfy, related to its efficiency in terms of low pollutant emission, energy saving and fuel flexibility. These constrains, imposed by the energy market fast changing requirements, give rise to the need of defining new and advanced solutions allowing to use, in the same unit, a broad range of energy carrying molecules (i.e. fuels), according to local and time market offer while preserving combustion efficiency and eco-sustainability. It is well assessed that recirculating heat and/or combustion products can have beneficial effects not only on flame stability but also on pollutants production. More specifically, in the last years several combustion modes inherently based on a strong recirculation process have been proposed and some of them found their way to commercial use. Typical examples are MILD, HiTAC and other regenerative burners (FLOX) [5-9] but also high intensity combustion devices have been proposed that rely on quite similar concepts [10-12]. One of the main limitations to the large deployment of recirculation based combustion devices is the difficulty in stabilizing small-scale systems due to the high heat losses. Previous considerations poses the problem of identifying suitable configuration and geometries ca-pable of realizing stable, efficient and clean combustion processes on small scale preserving the fuel flexibility needed and a reasonable degree of simplicity. In this study we considered a configuration that can be considered ancillary both for the study of recirculation systems and recirculation zone in a full-scale system as well as small-scale burner con-figuration. Following, experimental tests were realized in a small size burner characterized by a strong internal recirculation ratio, induced by a cyclonic fluid-dynamic pattern obtained by the ge-ometrical configuration of the reactor and of the feeding system (Fig. 1). The facility was designed to independently vary the mixture pre-heating temperatures and the mixture dilution levels [13, 14]. In this context the combustion performances and stability of the cyclonic burner are analyzed to point out advantages and possible drawbacks of the burner as well as suitable working conditions and performances. Results suggest that the cyclonic configuration represents a challenging choice to stabilize the oxidation process while containing the pollutants emission for a large range of pre-heating temperature - mixture dilution levels extending the burner operability conditions with re-spect to the ones expectable on the basis of chemical/thermodynamic features of the process.