Oxy-fuel combustion is an enabling technology for capture of CO2 from coal combustion, the economics of which depends strongly on the ability of the process to produce low NOX emissions. The literature contains many reports of lower NOX emissions from oxy-fuel combustion but the reasons for this are not fully understood. The objective of this work was to gain understanding of nitrogen evolution under pulverized coal oxy-fuel conditions. Pulverized coal was burned in a once-through, down-fired, laminar flow reactor. Nitrogen compounds and other combustion species were measured at the reactor centerline as a function of distance from the burner. Dry recycled flue gas was simulated with CO2 and O2 was added to form an oxy-fuel oxidizer. Oxy-fuel combustion measurements were compared to similar experimental data from air-fired cases. In addition, a detailed kinetic model was written and nodel predictions were compared to the experimental data. These comparisons gave insight into the mechanisms of nitrogen evolution under oxy-fuel conditions. The combustion model matched the experimental data well in many qualitative respects but failed to predict reburning reactions which are believed to be important in both air and oxy-fuel combustion. Model assumptions related to particle size and mixing may be responsible for this difference. Several mechanisms other than reburning are discussed with respect to their importance in the results. The effect of varying primary combustion zone stoichiometry (depth of staging) was investigated and it was found that oxy-fuel combustion, like air combustion has some depth of staging that produces minimum NOX. At minimum NOX conditions in this once-through experiment both air and oxy-fuel combustion converted a similar amount of fuel-bound nitrogen to NOX, however the minimums were at significantly different stoichiometries. Relative to air combustion, oxy-fuel combustion was found to exhibit higher concentrations of CO, NH3, HCN, and hydrocarbons, which indicates a more effective reburning environment exists in oxy-fuel combustion relative to air, even at higher primary stoichiometric ratios. This and other factors such as maximizing the amount of recycled NOX passing through the fuel-rich flame lead to the conclusion that oxy-fuel combustors should be operated at higher primary stoichiometric ratios than air combustors, which would conveniently also favor high fuel burnout.
College and Department
Ira A. Fulton College of Engineering and Technology; Mechanical Engineering
BYU ScholarsArchive Citation
Mackrory, Andrew John, "A Mechanistic Investigation of Nitrogen Evolution in Pulverized Coal Oxy-Fuel Combustion" (2008). All Theses and Dissertations. 1595.
coal, combustion, modeling, NOx, oxy-fuel, nitrogen