Effects of Co-flow on Jet Diffusion Flames: Flow Field and Emissions
Creators: Zamani, Milad
Publisher: name: University of Alberta Library
Year: 2023

<p>Industrial flaring is a notable global contributor to carbon dioxide emissions and other key pollutants, such as black carbon, oxides of nitrogen, and unburnt hydrocarbons. Introducing a separate assisting fluid near the base of these flames affects their hydrodynamics, thermodynamics, and chemistry, which in turn affects their overall efficiency and emissions. The extent of these three effects can differ depending on the injection geometry, the composition of assisting fluid (inert gases, air, steam, or atomized liquid water), and the quantity of the assisting fluid added. This study comprises three connected experimental investigations to address the effects on efficiency, emissions, and stability of lab-scale co-flow jet diffusion flames in either a co-annular burner or a slot burner.</p>

<p>In Investigation I, a burner was constructed of two concentric tubes allowed for various generic burner geometries, where different fuels (methane or propane) and co-flow assisting fluids (air, steam, or inert gases) flowed through the annular space and the center tube, respectively. The effects of the composition and flow rate of the fuel and assisting fluid, as well as the burner head geometry, were investigated in terms of carbon conversion efficiency (CCE) and emission indices of black carbon and oxides of nitrogen. Adding low flow rates of the assisting fluid co-flow significantly reduced the black carbon emission, which was not related to the CCE collapse with the main flame blow-off occurring at higher flow rates of assisting fluid. Moreover, any changes that resulted in a higher/lower characteristic flame temperature increased/decreased the emission of oxides of nitrogen. These results showed that, irrespective of the fuel or assisting fluid composition (other than the oxygen-enriched case), there was a range of assisting fluid flow rates, where the CCE was approximately 100 %, while the emissions of black carbon and nitrogen oxides were highly suppressed.</p>

<p>To focus on the effects of low flow rates of water as the assisting fluid on emissions, in Investigation II, a burner was constructed of a contoured nozzle with an inner tube at the center. Different fuels (methane, propane, or methane-propane mixture) and co-flow assisting fluids with the same chemical composition (steam or atomized deionized water) flowed through the annular space and the center tube, respectively. The results revealed that water addition suppressed the emission of black carbon and nitrogen oxides more significantly compared to the same mass of steam addition, which is due to the stronger thermal effect of water as the assisting fluid.</p>

<p>In order to explore the hydrodynamic effects of assisting fluid on the stability of the diffusion flames, in Investigation III, a multi-slot burner was designed and tested. This burner allowed for stabilizing normal and inverse diffusion flames, as well as the coexistence of both in various configurations with 2D optical access. To finalize the design dimensions and specifications, numerical simulations and non-reacting flow particle image velocimetry (PIV) tests were performed on single slots to get the exit velocity profile along their short and long side. After constructing the multi-slot burner, the coexistence of the normal and inverse diffusion flames was investigated by performing reacting flow PIV tests and overall emission measurements. The notable finding is that the inner air flow rate must be high enough to lift off the inner flame and enable fuel and air premixing to lower the total BC emissions.</p>