Oxygen-enhanced combustion: Theory and applications

In Part I, coflow flame experiments were performed to compare and evaluate the influence of flame structure on soot formation when operating under normal and inverse flame conditions. Flame structure was shown to influence soot formation in a similar fashion for normal and inverse flames when the effects of residence time were removed. The simple model previously discussed was modified to account for finite-rate chemistry and residence time effects, and was correlated with experimental data leading to the determination of the critical local temperature and critical local C/O ratio for soot inception in ethylene flames. The presence of appreciable oxygen at the location of maximum temperature was investigated using a flame code with detailed chemistry. The mechanisms responsible for O2 at xTmax in high Zst flames were determined and explained. This phenomenon was attributed to a shifting of the location of maximum temperature relative to the location of oxygen depletion, and the temperature shift was explained by considering the variations in the heat release profile at high Zst .;A second numerical investigation was also conducted to evaluate the significance of the local critical C/O ratio as a parameter describing soot-free conditions, the role of oxidizing species at this location, and changes that occur in the chemical pathway to the formation of soot precursors at high Zst . The critical local C/O ratio was shown to correspond to the edge of the radical pool for flames of any Zst, and oxidizing species did not appear to accelerate soot precursor oxidation at high Zst as previously thought. A reverse pathway analysis was used to determine the dominant chemical pathway leading to the formation of soot precursors. At high Z st, a key soot precursor formation step was observed to reverse leading to the destruction of propargyl (C3H3) to form acetylene (C2H2) as opposed to benzene (C6H6) and phenyl (C6H5). The existence of soot-free flames at long residence times was attributed to this phenomenon.;In Part II of this work, a form of OEC currently being considered as an enabling technology for carbon dioxide capture from pulverized coal (PC) utility plants, termed oxy-fuel combustion, was considered. Oxy-fuel combustion utilizes oxygen and recycled flue gases (RFG) as the oxidizer instead of air, therefore the concentration of oxygen in the coal carrier stream, as well as any other concentric stream or quiescent environment, is a variable. The viability of oxy-fuel combustion can be enhanced by its ability to reduce capital and operational costs by, for example, lowering the emissions of nitrogen oxide species (NOx) in situ. Studies have demonstrated that oxy-fuel combustion can lower NOx emissions by as much as 70% when compared to conventional coal/air combustion, largely due to the reduction of recycled NOx to molecular nitrogen when interacting with hydrocarbon species in the flame.;This work investigates the potential for reduced NOx emissions under oxy-fuel conditions through variations in the gas composition of the fuel carrier and concentric oxidizer streams. Nitric oxide (NO) emissions were measured during the combustion of PC and PC/sawdust mixtures under air-fired and oxy-fuel conditions. The effects of excess oxygen, secondary oxidizer swirl, carrier gas flow rate, and sawdust cofiring on NO emissions were investigated. Under oxy-fuel conditions, the effect of varying the compositions of the carrier gas and concentric oxidizer streams on NO emissions was also investigated. Under the optimal oxy-fuel conditions, NO emissions were reduced by 20% when compared to air-firing. Cofiring coal with sawdust that contained less fuel bound nitrogen did not reduce the NO emissions under air-fired or oxy-fuel conditions. Changing the adiabatic flame temperature by varying the oxygen concentration in the concentric oxidizer stream did not significantly influence NO emissions until the temperature was too low and flame instabilities were observed. When increasing the oxygen concentration in the coal carrier gas a critical local stoichiometric ratio was observed that led to increased NO emissions. (Abstract shortened by UMI.)...