OCR Text |
Show flame temperature by the addition of inert diluents (e.g., cooled recycled combustion products or water addition) minimize thermal NO formation but have a minor impact upon fuel NO production. Staged heat release (staged combustion) provides the most effective NO control technique for nitrogen-containing fuels because fuel NO formation is mainly dependent upon local stoichiometry. It can be accomplished either by separating the combustion chamber into two zones and dividing the total combustion air into two streams, or by appropriate burner design which promotes localized fuel-rich conditions. The basis for minimizing fuel NO formation is independent of the fuel type, and requires the existence of a fuel-rich primary combustion zone to maximize the conversion of fuel nitrogen to molecular nitrogen since the fate of fuel-bound nitrogen is strongly controlled by the reactant stoichiometry. Many studies have shown that under premixed fuel-rich conditions the efficiency of conversion to N« Increases significantly. Thus there are two fuel nitrogen reaction paths leading to the production of N« or NO, namely: Path A. Fuel-lean XN + Oxidant + NO + Path B. Fuel-rich XN + .... •+ N2 + .... The objective of staged combustion emission control techniques is the provision of conditions which maximize N« production via Path B, Two factors of practical importance are the residence time and the stoichiometry required to maximize N2 production in the fuel-rich primary zone, If the residence time is insufficient then the original fuel nitrogen specie will exist in the gaseous state as some XN compound which can be converted to NO in the second stage heat release zone. The stoichiometry required to achieve minimum XN concentrations at the exit of the primary stage will be determined by (.1) the rate of evolution of nitrogen specie 7-3 |