NOx Dependency on Residence Time and Inlet Temperature for Lean-Premixed Combustion in Jet-Stirred Reactors

Abstract

The effect of the residence time variation on NOx formation in high-intensity, lean-premixed (LP) methane combustion is explored through experiments conducted in a high-pressure jet-stirred reactor (HP-JSR) operated at 6.5 atm pressure. The residence time is varied between 0.5 ms and 4 ms, holding the measured reactor recirculation zone temperature constant at 1803 K. Air preheat is not used. The results indicate a minimum NOx level of 3.5 ppmvd (15% O2) for reactor mean residence times between 2 and 2.5 ms. As the residence time is reduced from 2.0 ms to 0.5 ms, the NOx increases, consistent with a spreading of super-equilibrium concentrations of free-radicals throughout the reactor. For the shortest residence times examined, PSR modeling agrees with the NOx measurements. At long residence times, (i.e., above 2.5 ms), the measured CO behavior indicates the super-equilibrium free radicals, and thus the rapid NOx production, are confined mainly to the jet zone of the reactor. For the long residence time range, the measured NOx increases with increasing residence time, and is significantly less than the PSR predictions. A simple two-zone model of the HP-JSR is used to interpret and evaluate the NOx formation. Experiments exploring the effect of inlet temperature on NOx are conducted in an atmospheric pressure, methane-fired, jet-stirred reactor (A-JSR). The reactor temperature is held constant at 1788 K, and the inlet mixture temperature is varied between the no-preheat case and 623 K. These experiments show that increasing the inlet air temperature over the full range tested decreases the NOx by about 30%. Several explanations are offered for the behavior. For both reactors, i.e., the HP-JSR and A-JSR, single inlet jet nozzles are used. The results lead to a practical conclusion that very low NOx levels can be achieved for combustion in strongly back-mixed reaction cavities adjusted to optimal residence time and inlet temperature.

NOX as a Function of Fuel Type: C1-to-C16 Hydrocarbons and Methanol

“…Combustion temperature is measured with a 0.127 mm type-R thermocouple coated with a ceramic compound (to prevent catalytic oxidation of CO, Hy, and hydrocarbons at the thermocouple surface) and corrected for radiation and conduction losses (typically, about a 35°C correction). Rutar et al (1998) provide a detailed description of the gas temperature correction analysis. Uncertainty in the combustion temperature measurements is judged to be ±20°C.…”

Aerothermodynamic cycle analysis of a dual-spool, separate-exhaust turbofan engine with an interstage turbine burner

“…In general, the N O mechanism above can be decoupled from the fuel oxidation computation because the N O formation becomes significant only after the fuel oxidation is complete. Despite the fact that the residence time is short, approximately 1 to 3 msec [44], it is assumed here that the N 2 , O 2 , O, H, and OH concentrations are at their equilibrium values and N atom is in steady state.…”