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Show Paper #7 NITRIC OXIDE FORMATION FROM FUEL-N IN STAGED COMBUSTION FOR LIQUID FUELS ABSTRACT Jacek Gromulski Studsvik Energy S-611 82 Nyk6ping, Sweden Nitric oxide formation from fuel nitrogen in staged combustion is investigated to assess the impact of fuel properties on the formation and control of ~o emissions. Effects of operating conditions such as stoichiometric ratio, fuel types, and amount of fuel nitrogen were examined experimentally for six liquid fuels with nitrogen content from 0.04 to 9.5%. Exhaust NO emissions were found to be directly related to the amount of oxidizable nitrogen species leaving the first stage. NO, HCN and NH3 concentration were measured in the fuel-rich zone of the staged combustor as a function of stoichiometry for three liquid fuels. Similar characteristics were observed for all liquid fuels. As the first stage stoichiometry was reduced, NO concentrations at the first stage exit decreased, and below SR1=0.8±0.05 HCN and NH3 concentrations increased. Thus, the total fixed nitrogen (TFN=NO+HCN+NH3) concentration passed through a minimum. Both staged and excess air combustion could be predicted from the TFN concentration data exiting the first stage of combustion. INTRODUCTION In a traditional boiler, fuel and combustion air are introduced through a burner located on the front firing face. The air flow rate is typically 5 to 30% in excess of stoichiometric requirements and the resulting flame reaches peak temperatures on the order of 2000K. A SUbstantial quantity of nitrogen oxide is formed in the high temperature regions of the flame. As the combustion gases flow down the boiler and heat is removed the equilibrium NOx concentration rapidly falls. At adiabatic flame temperatures equilibrium NOx concentrations may reach several thousand ppm, while at the boiler stack the equilibrium concentration falls to well below 1 ppm. Unfortunately, the kinetic rates for the prevailing nitrogen chemistry are too slow to permit the NOx concentration to follow the drop predicted with equilibrium assumptions. That is, the NOx concentration is frozen at the high levels produced in the active flame. The experimental portion of this work was conducted under fuel rich conditions. Fuel rich conditions are known to be important in the reduction of NOx emissions from a variety of combustion processes. An important case is that of staged combustion of liquid fuels with high content of nitrogen. Staged combustion is characterized by an initial fuel rich combustion zone followed by a fuel lean stage for fuel burnout. The nitrogenous species which lead to NOx emissions 1 |
