An Instability of Diluted Lean Methane/Air Combustion: Modeling and Control

Abstract

In order to study possibilities of Model Predictive Control (MPC) to low temperature combustion, the authors performed a numerical study of combustion with a lean highly diluted methane/air mixture in a perfectly stirred reactor using a detailed chemical kinetic model. Chosen conditions are the following: equivalence ratio 0.6, residence time of mixture in reactor 0.5 s; molar fraction of Nitrogen 0.9, temperature of incoming unburned gases and reactor 1100 K, and heat loss coefficient 2 × 10−3 cal/(cm2Ks). At these conditions strong oscillations are predicted in agreement with previous experimental findings of M. de Joannon et al. (2004). It is found that, at low temperatures (<1300 K), reactions forming CH3O and CH3O2 from CH3 had a strong influence on the oscillations. Once these reactions were subtracted from the kinetic model, no oscillations were observed. A virtual MPC suppressed the oscillations at low temperatures successfully and demonstrated extremely low NO and N2O emissions (<0.1 ppm).