Mild combustion is an emerging technology in several fields of practical applications, from material treatment to energy conversion as well as to pollutant abatement. In a survey of potential keys for the mitigation of environmental problem, it represents a flexible and clean process that is the result of a trade-off in the optimization of fuel conversion with respect to efficiency (in term of energy saving and pollutant abatement) while not requiring drastic changes in the configuration of traditional plants. As well assessed in literature, Mild combustion is rigorously defined such as a process for which "the inlet temperature of the reactant mixture is higher than mixture self ignition temperature whereas the maximum allowable temperature increase with respect to inlet temperature during combustion is lower than mixture self ignition temperature". In other words, Mild combustion occurs when highly pre-heated and very diluted reactants are taken into account. From practical point of view, such conditions can be realized in several ways on the basis of the typology of system considered. It is not possible, here, to go inside such a topic due to lack of the room and also because it has been extensively discussed in literature. It is only worthwhile to highlight that each configuration can yield to its own peculiarity, even though the main features of Mild Combustion can be generally recognized in all the configurations, as will be clear in the following. In this chapter the attention will be focused on non-premixed combustion processes, thus on configurations where the fuel and the oxidant flows are fed separately, then mix and react. A combination of both heating and dilution of oxidant and/or fuel yields a not premixed combustion process which is named Hot Diluted Diffusion Ignition (HDDI) [2, 4] when oheating contributes significantly to the creation of an oxidative structure in the sense that no combustion process occurs without it; odilution is so intense that the maximum temperature attainable inside this structure is so low that, in turn, it affects significantly its placement in the mixture fraction domain, the structure itself and the physical and chemical kinetics when compared to a diffusion flame process. All or part of these combustion characteristics are expected to be obtained when the self- ignition temperature of the stoichiometric mixture is lower both than the temperature of at least one reactant and than the adiabatic flame temperature of the stoichiometric mixture. As matter of fact this is the simplest "a priori" definition of a HDDI process even though it catches only approximately all the features of the process. A deeper insight and a more defined domain, in which the HDDI process occurs, is obtained by the analysis of both the temperature mixture fraction plots and physical/numerical models for assigned process pressure and strain rate when the combination of both heating and dilution is fixed for a specific reactant. Four combinations of these last parameters are possible when they are assigned only for one reactant. The HDDI process has been investigated systematically only for the first two conditions even though interesting features have been hypothesized also for the last two. In particular the processes occurring with the constraint imposed by the first condition have shown that the aforementioned "a priori" definition is the proper one in identifying a domain where the oxidative structure is completely different from the diffusion flame one.
HDDI Mild Combustion - chapter 7th in Alternative Ignition Systems
de Joannon M;Sabia P;
2009
Abstract
Mild combustion is an emerging technology in several fields of practical applications, from material treatment to energy conversion as well as to pollutant abatement. In a survey of potential keys for the mitigation of environmental problem, it represents a flexible and clean process that is the result of a trade-off in the optimization of fuel conversion with respect to efficiency (in term of energy saving and pollutant abatement) while not requiring drastic changes in the configuration of traditional plants. As well assessed in literature, Mild combustion is rigorously defined such as a process for which "the inlet temperature of the reactant mixture is higher than mixture self ignition temperature whereas the maximum allowable temperature increase with respect to inlet temperature during combustion is lower than mixture self ignition temperature". In other words, Mild combustion occurs when highly pre-heated and very diluted reactants are taken into account. From practical point of view, such conditions can be realized in several ways on the basis of the typology of system considered. It is not possible, here, to go inside such a topic due to lack of the room and also because it has been extensively discussed in literature. It is only worthwhile to highlight that each configuration can yield to its own peculiarity, even though the main features of Mild Combustion can be generally recognized in all the configurations, as will be clear in the following. In this chapter the attention will be focused on non-premixed combustion processes, thus on configurations where the fuel and the oxidant flows are fed separately, then mix and react. A combination of both heating and dilution of oxidant and/or fuel yields a not premixed combustion process which is named Hot Diluted Diffusion Ignition (HDDI) [2, 4] when oheating contributes significantly to the creation of an oxidative structure in the sense that no combustion process occurs without it; odilution is so intense that the maximum temperature attainable inside this structure is so low that, in turn, it affects significantly its placement in the mixture fraction domain, the structure itself and the physical and chemical kinetics when compared to a diffusion flame process. All or part of these combustion characteristics are expected to be obtained when the self- ignition temperature of the stoichiometric mixture is lower both than the temperature of at least one reactant and than the adiabatic flame temperature of the stoichiometric mixture. As matter of fact this is the simplest "a priori" definition of a HDDI process even though it catches only approximately all the features of the process. A deeper insight and a more defined domain, in which the HDDI process occurs, is obtained by the analysis of both the temperature mixture fraction plots and physical/numerical models for assigned process pressure and strain rate when the combination of both heating and dilution is fixed for a specific reactant. Four combinations of these last parameters are possible when they are assigned only for one reactant. The HDDI process has been investigated systematically only for the first two conditions even though interesting features have been hypothesized also for the last two. In particular the processes occurring with the constraint imposed by the first condition have shown that the aforementioned "a priori" definition is the proper one in identifying a domain where the oxidative structure is completely different from the diffusion flame one.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
