Stability of Flames Close to Auto-Ignition Temperatures Generated by Extreme Separated Gas-Air Inlets

Stabilization of the combustion of natural gas in high-temperature processes by using the auto-ignition of the fuel when mixed with highly preheated air is well known and has found application on many occasions. Reasonably strong internal flue gas recirculation not only reduces nitric oxides emissions and increases convective heat transfer rates, but reduces local flame temperatures such that the flames become almost invisible for a human eye. This combustion regime is called flameless oxidation. Gasunie’s interest in this technique of flameless oxidation has two aspects. First, it must be clear which geometrical restrictions and flow conditions/disturbances in the oven or furnace have to be taken into account. Secondly, the use of this principle requires the auto-ignition of the fuel. This raises the question as to the stability of the combustion at or near the limits for auto-ignition. The study which is presented here reports on the stability of the oxidation process at these limiting conditions. These conditions are minimum load to the combustion system and minimum temperature in a combustion chamber. The stability has been determined using some “burner”/furnace combinations in which the distance between nozzles for air and natural gas have been varied.