NO emission from non-premixed MILD combustion of biogas-syngas mixtures in opposed jet configuration

In this paper NO emission from MILD combustion of the mixture biogas-syngas is deeply elucidated, five NO routes were considered, specifically: thermal, prompt, NNH, N₂O and reburning. Several operating conditions are studied namely: fuel mixture composition, oxygen concentration in the oxidizer and injection velocity or strain rate. Biogas is modeled by a mixture of methane and carbon dioxide; while, syngas is considered to be composed by hydrogen and carbon monoxide, this gives a fuel mixture of CH₄/CO₂/H₂/CO. Volume of methane and hydrogen are varied alternatively from 0 to 50% in fuel mixture. Oxidizer is composed by O₂/N₂ mixture where oxygen volume is increased from 4 to 21%. Finally, injection strain rate is varied from apparition to vanishment of combustion. Atmospheric pressure is considered with constant fuel and oxidizer injection temperatures of 300 K and 1200 K respectively. Chemical kinetics of such complicated system is handled by a composed mechanism from the USC C₁–C₄ and the Gri 2.11 N-sub mechanism. It is found that under MILD regime, temperature intervals and levels are enhanced by hydrogen compared to methane. Furthermore, temperature levels keep relatively low which guarantees MILD regime. Contrariwise, when oxygen increases in oxidizer, temperature grows up rapidly and the MILD regime disappears. However, if strain rate augments, temperature shows a steep increase then reduces monotonically. It is observed that for low methane volume in the fuel mixture, NNH route dominates NO production. Whereas, when CH₄ increases, the prompt route is enhanced and exceeds NNH one at a methane volume of 12%. When hydrogen increases, prompt and NNH routes are enhanced with a domination of the prompt route until 44% of hydrogen volume. Oxygen increasing in the oxidizer improves thermal mechanism which surpasses prompt one at 17% of oxygen volume and governs NO production. Globally, the third most important route in NO production is the reburning one which is enhanced by all parameters except strain rate.