| OCR Text |
Show American Flame Research Committee 1992 Fall International Symposium October 19-21, 1992 The hydrocyanide funher reacts to form N2 at a rate that is dependent upon the fuel equivalence ratio and temperature of the fuel-rich flame zone. Flue Gas Recirculation Under lean conditions, NO is fonned primarily by the Zeldovich mechanism. Thus, reducing flame temperature in the lean stage effectively controls the NO formation rate. An effective method of reducing combustion temperature is to introduce inert substances (e.g. recirculate flue gas) into the combustion zone. These substances absorb a fraction of the energy as combustion products are heated from their initial temperature, resulting in reduced flame temperature. Secondary Zone Combustion NO Formation Rates FOR inttoduces principally N1, H20, and CO2 to the combustion zone. All of these species absorb energy while being reheated to flame temperature. The process may be even more effective if some of the molecules of ~ and H20 dissociate as they are heated to high temperatures. For most FGR applications, the amount of combustion products that can be recycled is limited by the need to pfe~rve flame stability. The StAR burner, however, has a special design feature whereby the fuel and primary air are introduced into the fuel-rich flame zone in a manner that significantly extends the amount of recyclable flue gas before flame instability occurs. The actual limitation of the amount of FGR is set by the capacity of the FGR pump rather than by flame stability. Thennal NO formation in the fuel-lean burn-out zone is reduced partly by the addition of inert gases (FOR) to the pyrolysis zone, and partly by the unique secondary air nozzles design. The combustible products of the fuel-rich flame ate intercepted in the furnace by secondary air jets that entrain fuUy burned and cooled combustion products before they mix with the products of the fuel-rich flame and burn as a secondary flame. In this mode of combustion, both the peak temperature and the O2 concentration of the oxidant gas are reduced with the result of strongly diminished thennal NO formation. To evaluate the relative imponance of these burner concepts, local rates of fonnation of NO in flame volume elements were calculated from in-flame measurements, following Sadakata and Beer (1976). Close to the exit of the primary stage, negative and small positive rates of NO fonnation were computed. High concentrations of C~ measured in this region indicated that destruction of NO through the "reburn route" was responsible for the negative values. These rates indicate that the staging air concept plays an imponant role in suppressing NO fonnation. |
