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).

Keywords:

• Combustion instability
• Heat loss
• Limit cycle
• Model reduction
• NOx
• Predictive optimization

ACKNOWLEDGMENT

The authors acknowledge valuable discussions with Prof. Eliseo Ranzi (Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, Italy) and Dr. Mara de Joannon (Istituto di Ricerche sulla Combustione, C.N.R., Napoli, Italy), and financial support from the Deutsche Forschungsgemeinschaft (DFG) to Sonderforschungsbereich (SFB) 686 at RWTH-Aachen University.