Formation of NOx and burnoff of co during thermal quenching of the products from combustion in a thermally stabilized burner

Abstract

Thermally stabilized combustion in a refractory tube can be controlled to produce minimal concentrations of NOx, but at the expense of moderate concentrations of CO. The objective of the present investigation was to determine if, by cooling the products of combustion rapidly, the concentration of CO could be reduced while avoiding the formation of additional NOx. In the experimental work, premixed ethane and air were burned in passage through seven 9.5-mm round channels in a ceramic block. In this burner, volumetric rates of heat release due to combustion greater than 2.0×108 W/m3 were achieved. The products of combustion were then passed in series through stainless steel tubes of the same diameter immersed in boiling water. The composition of the gas was determined at a number of locations along one of the cooling tubes. Twenty-one free-radical reactions were used to model the chemical behavior of the gas during this process of cooling. The two-dimensional fields of velocity and temperature, upon which the chemical behavior depends critically, were obtained from previous calculations. Both the measurements and the computations showed the mole fraction of NOx to be frozen at the value near the entrance of the cooling tube. The formation of prompt NOx is negligible in this burner owing to minimal back-mixing. Hence, the total formation of NOx is proportional to the time of residence between the flame front and the exit of the burner, which can be reduced to as low as 2.0 ms on the mean, resulting in concentrations as low as 10 ppm in the effluent. For lean mixtures the CO was found to continue oxidizing during the process of cooling, leaving, for an equivalence ratio of 0.80, as few as 90 ppm (.009%) in the effluent. Forabove 0.88, the greater initial amount of CO resulted in its incomplete oxidation during the cooling. Owing to the use of forced convection inside tubes, a very high rate of heat transfer from the burned gas to the boiling water was achieved. Thus, the overall process described herein is very efficient thermally as well as minimally polluting.