Premixed Laminar Flames in Mild Combustion Conditions

Over the last years high efficiency and reliability, abatement of pollutants as well as energy saving have been the main requirements for the development of new combustion processes. In this background one of the most promising technologies is the combustion process that is carried out with high inlet temperatures and high dilution degrees of reactants. This combustion process is named in several ways such as Mild Combustion, High Temperature Combustion and High Temperature Air Combustion. Although several industrial plants apply such a combustion mode, especially inside the atmospheric-pressure furnaces, Mild Combustion mode has not been used in gas turbine combustion chambers yet. The main advantages of Mild Combustion concern the improving of stability of combustion zone as well as the decreasing of pollutants emission, such as soot and NOx, and of thermal stresses, mainly due to hot spots in traditional combustion chambers. Inlet mixture preheating, characteristic of this innovative combustion technology, can be obtained by using the exhausted gas enthalpy either by means of an external heat exchanger or by means of high internal recirculation systems. A preliminary numerical analysis has been realized in order to identify the combustion regimes establishing when the recirculated hot gas meets the fresh mixture fed to the combustion chamber and to verify the possibility of obtaining a stable combustion process even for ultra-lean mixtures. As matter of fact a deflagration process can not stabilized for very lean mixtures outside the flammability limits. Nevertheless, oxidation may occur if the mixture is sufficiently heated. Several experimental studies have stated the feasibility of a oxidation process based on the idea of heating lean mixtures by means of high temperature inert flow in laminar or turbulent counter-diffusion reactor. In this work the zone where the hot flue gas are recirculated towards the fresh reactants have been schematized by means of the counter-flow configuration and it has been studied by means of numerical analyses using the Oppdif application of ChemKin 3.7 software and the methane oxidation kinetic model from the Gas Research Institute (GRI).