On By-Product Flames with Implications Towards Emission of Organics During Waste Incineration

Abstract

A three-step reaction mechanism is employed to study the production and release of by-products in flames. The emphasis is towards understanding the production of organic by-products that are formed by vapor-phase reactions during, for example, waste incineration. The reaction mechanism includes (1) reaction of the fuel with an oxidizer to produce products plus a radical that is needed to form the by-product; e.g., H or Cl, (2) reaction of the radical with the fuel to generate a by-product, and (3) consumption of the by-product with an oxidizer to form products. The resulting problem is solved by high-activation-energy asymptotic analysis and the solution identifies the flame temperature and flame location as well as the amount of by-product at the low-temperature boundaries of the by-product generation and consumption zones. These data indicate conditions where the by-product will be released into the products of combustion, either by leakage through the oxidizer side or by transport into the cooler, nonreactive region of the fuel side. Results show that the amount of by-product increases as the Damkohler number for the by-product consumption reaction, Da 3 , decreases, and there exists an extinction Damkohler number, Da 3,E , below which a high amount of by-product exists in the combustion products. The effects of reactivity of the generation and consumption reactions on by-product production are also discussed. The net by-product production is found to be, to a large extent, independent of the Lewis number of the radical, Le R , for Le R < 1. On the other hand, the effect of the Lewis number of the by-product, Le S , is more complex; for fuel-air flames the by-product leakage, flame temperature and Da 3,E display a non-monotonic dependence on Le S . This result is explained by considerations of enthalpy loss via by-product transport away from the oxidizer and its subsequent effect on flame temperature and reaction intensity for the by-product consumption reaction. In addition to studying fuel-air flames, diluted fuel-oxygen flames are studied wherein the air side nitrogen is substituted into the fuel such that the flame temperature is the same as for fuel-air flames. The relative flame locations are different, however, and this difference is found to dramatically reduce the production of the by-product. This result is a consequence of a changing flame structure. For the fuel-air flame, the temperature gradient in the oxidizer side of the reaction region is much steeper than that in the fuel side. The generation region, which is located at the fuel side of the oxidation region, is then much broader than the consumption region located on the oxidizer side of the oxidation region. Therefore, the residence time for the generation reaction is much larger than that of the consumption reaction, leading to a higher generation rate and a lower consumption rate of the by-product. When the inert gas is shifted to the fuel stream, the flame is located on the fuel side of the stagnation plane so that all conditions are reversed This then implies conditions that make it more difficult to generate, and relatively easy to oxidize by-product. Consequently the by-product concentration is much lower for the second case.