Modeling of Low NOx Combustion of Coal Gas with High Temperature Air from a Multi-jet Burner

Numerical simulation was carried out on the high temperature air combustion of coal gas in an industrial furnace with a multi-jet burner. The furnace was a rectangular chamber of 800 mm × 800 mm × 1400 mm. A circular fuel jet of a diameter of 10 mm is at the center of the wall. 5 circular air jets equably distributed around the fuel jet with different straddle angles. A PDF (probability density function) combustion model based on a ß function was selected to simulate the gas combustion combined with the standard k-¿model. The radiation was simulated by a discrete ordinates method. The NOx emission was simulated by thermal NOx model. The effect of the distance between the fuel and air jets on the combustion characteristics was discussed in this paper. The results showed that there was the recirculation zone in the furnace due to the interaction of the jets. The recirculation of the flue-gas changed the local distribution of the fuel and oxygen in the furnace and then influenced the local NOx generation. When the distance between the air and fuel jets increased, the mixing of the fuel and air could be effectively delayed and an obvious recirculation zone appeared between the air jets and fuel jet that reduced the local oxygen concentration. More fuel would be burnt in the low oxygen region and combustion zone and the flame volume were both increased which made the temperature distribution became more equable and local high temperature was suppressed. When the dimensionless distance between the air and fuel jets was 2.5, a minimum NOx emission of 45 ppm was achieved at 15% O2.