Operational, Combustion, and Emission Characteristics of a Small-Scale Combustor

This article examines the operational, combustion, and emission characteristics of a small-scale combustor. Flue-gas composition data and hydroxyl radical chemiluminescence (OH*) imaging are reported as a function of the excess air coefficient (lambda), which in the present configuration implies also changes in the inlet air velocity. For two of these combustor operating conditions, spatial distributions of temperature and of O-2, CO2, unburned hydrocarbons, CO, and NOx concentrations are also reported. The OH* images showed that as lambda increases the main reaction zone moves progressively closer to the burner presumably due to the increase in the central jet momentum, which leads to a faster entrainment of fuel and burnt gases, and due to the increase in the oxygen concentration in the recirculated flue-gas. The OH* images also reveal that the structure of the main reaction zone and the combustion regime change with lambda. For low values of lambda the reaction zone is uniformly distributed over a relatively large volume of the combustor (flameless combustion, also known as MILD combustion, HiTAC, or colorless distributed combustion), whereas for high values of lambda, the OH* images suggest and still photographs confirm the presence of a flame front located at the strong shear region between the central jet and the external recirculation zone (conventional lean combustion). The present combustor yields very low NOx (< 10 ppm @ 15% O-2) and CO emissions (< 12 ppm @ 15% O-2) for all conditions studied, which is attributed to the suppression of the thermal mechanism brought about by the flameless oxidation and conventional lean combustion modes. Finally, the detailed measurements made inside the combustor for the two operating conditions, a flameless oxidation condition and a conventional lean combustion condition, confirmed the observations based on the OH* images.