CO2 effects on near field aerodynamic phenomena in 40kW, co-axial, oxy-coal, turbulent diffusion flames

Abstract The effects of oxy-coal combustion on the near-field aerodynamics of simple, co-axial turbulent diffusion flames have been investigated. Experiments were carried out in a specially designed 40 kW (100 kW nominally) combustion test rig. Results from two sets of experiments are presented, with each set using once-through CO2, without H2O, plus added O2 for the oxidant stream. The first set consists of screening experiments that revealed: (i) how replacement of dilution N2 by CO2 (as in oxy-coal combustion) affected NOx emissions at equivalent adiabatic flame temperatures; (ii) how, for oxy-coal combustion at constant P O 2 , primary , changes in P O 2 , secondary affected lengths of both the standoff distance and the luminous zone, and (iii) how the luminosity of oxygen-enriched air flames compared to that of aerodynamically similar oxy-coal flames at similar adiabatic fame temperatures. These screening experiments also allowed for the evolution of a methodology for quantifying flame standoff distance from video images. This methodology involved photo-image processing and probability density functions (PDFs) that represented observed fluctuations in the near-field aerodynamic region. The second set consists of final experiments after the data processing techniques had been refined. Results presented here focus on the relationship between the flame shapes, as seen by an observer, to the PDFs used to quantify the fluctuating flame standoff distance. The latter was taken to be an essential characteristic of the near-field aerodynamics of oxy-coal flames. Flame images (yielding practical, but qualitative information) and the concomitant PDFs (yielding fundamental, and quantitative data) are presented showing the separate effects of primary P O 2 , secondary P O 2 , transport diluent composition, furnace wall temperature, and secondary stream preheat temperature. A simple model in which ignition was controlled by the rate of molecular diffusion of O2 through a gas film composed of the primary jet transporting fluid appeared to correlate some of the data. These experimental data provide not only a quantitative insight into mechanisms of ignition of coal in turbulent oxy-coal flames of practical relevance, but also (with uncertainty quantification) a quantitative basis for validation of future detailed simulations of this process.