Effect of Air-Fuel Ratio on Temperature Distribution and Pollutants for Biogas MILD Combustion

Abstract

This paper examines the effect of air-fuel ratio for Moderate or Intense Low oxygen
Dilution (MILD) combustion using a bluff-body burner. Exhaust gas recirculation was
used to dilute the oxidizer stream prior to the combustion chamber. A low-calorie
biogas fuel which consists of 60% methane and 40% carbon dioxide were used in the
simulations using a Reynolds-averaged Navier–Stokes model with the realizable k-ε
turbulence model. The chamber temperature distribution was found to be in small
ranges and almost homogeneously distributed, verifying that MILD conditions were
attained. The performance was evaluated based on the level of pollutants (Unburned
hydrocarbons (UHC) and carbon-mono oxide (CO)) produced and measured in the
exhaust gas. Slightly lean conditions produced negligible pollutants with some excess
oxygen measured in the exhaust gas. Under rich conditions, UHC and CO were
produced, but when synthetic air containing oxygen with a mole fraction of 7% was
used as the oxidizer instead of ordinary air, these levels were significantly reduced.