Combustion intermediate species

In the complicated reaction networks involved in fuel decomposition and oxidation, intermediate species indicate the presence of different pathways that may be important under specific combustion conditions. While the final products of hydrocarbon/air or oxygenate/air combustion, commonly water and carbon dioxide, are of increasing importance with respect to combustion efficiency—with the perception of carbon dioxide as a... 

In contrast to NaZSM-5 zeolite, introduction of CoZSM-5 or HZSM-5 zeolite in the reaction system shifts the "light-off" temperature and modifies the chemistry now not only NO but Nj is formed. Hence, some intermediate species required for Nj formation must be stabilized on the catalyst surface. The "light-off"temperature shifts observed with CoZSM-5 and HZSM-5 catalysts may result from the enhanced redox capacity provided by these catalysts or from the NOj/NO equilibrium achieved more readily than with NaZSM-5. Moreover, equilibrium is approached at a somewhat lower temperature over CoZSM-5 than HZSM-5, and much lower than with the empty reactor (see Fig. 1 of Ref. lOl.The decomposition reaction of NOj into NO -t- occurs readily on these catalysts and the "light-off" temperature of both combustion and SCR is lower in comparison with that of the homogeneous reaction. 

Bollig, M., H. Pitsch, J. C. Hewson, and K. Seshadri. 1996. Reduced n-heptane mechanism for nonpremixed combustion with emphasis on pollutant relevant intermediate species. 26th Symposium (International) on Combustion Proceedings. Pittsburgh, PA The Combustion Institute. 729-37. 

Computational and experimental methods clearly benefit from a symbiotic relationship in combustion studies. Theoretical calculations can propose important pathways to yield empirically observed intermediates by providing reaction energies and rate coefficients of elementary reactions, thereby guiding experiments. Moreover, theoretical calculations can potentially fill some gaps caused by limitations in experimental approaches the vast majority of analytical techniques fail to distinguish between structural isomers and to identify short-lived intermediate species, both of which are important objectives in delineating overall combustion behavior. Finally, modeling can identify species to look for experimentally. 

Mechanism for Deriving Energy. The mechanism by which propulsive energy is derived from propellant systems containing metals and their compounds is somewhat different from that of conventional liquid propellant systems. For hydrazine and nitrogen tetroxide, for example, their combustion leads to the formation of N2, H20, and H2 through a relatively simple series of intermediate species ... 

As previously mentioned, LIF is the method of choice for detection of the transient intermediate species which form the keys to the chemistry of combustion. 

Greater than equilibrium concentrations of intermediate species have been observed in the combustion products of several reactant systems. Examples are the concentrations of ammonia in the products of the decomposition of hydrazine (32), the concentration of CH4 in ethylene oxide decomposition (33), nitric oxide and ammonia in the products of the reaction of hydrazine and nitrogen tetroxide (34), and chlorine monofluoride in the products of the reaction of hydrazine and chlorine pentafluorlde (35). 

When combustion of a well-mixed fuel-air mixture occurs, the fuel rapidly reacts with oxygen to form a number of unstable intermediate species (such as oxygen and hydrogen atoms, and OH and H2O radicais), which then proceed through a complicated chain mechanism to form CO2 and HiO. Some of these species undergo transitions that cause them to emit radiation whose wavelength falls within the blue region of the visible spectrum. The result is that the flame appears blue. 

For the last 20 to 25 years, computer modelling has been used increasingly to interpret combustion phenomena. When the chemistry is the main interest, for example, in predicting ignition delay times or predicting product profiles, large comprehensive mechanisms can be used which in theory cover every species possible in the total oxidation. In most treatments differential equations are written for the reactants, intermediate species and for the final products. These are solved by standard mathematical treatments and profiles of concentration - time produced for all species. This approach will be discussed in detail in Chapter 4. 

The main problem in the solution of non-linear ordinary and partial differential equations in combustion is the calculation of their trajectories at long times. By long times we mean reaction times greater than the time-scales of intermediate species. This problem is especially difficult for partial differential equations (pdes) since they involve solving many dimensional sets of equations. However, for dissipative systems, which include most applications in combustion, the long-time behaviour can be described by a finite dimensional attractor of lower dimension than the full composition space. All trajectories eventually tend to such an attractor which could be a simple equilibrium point, a limit cycle for oscillatory systems or even a chaotic attractor. The attractor need not be smooth (e.g., a fractal attractor in a chaotic system) and is in some cases difficult to compute. However, the attractor is contained in a low-dimensional, invariant, smooth manifold called the inertial manifold M which locally attracts all trajectories exponentially and is easier to find [134,135]. It is this manifold that we wish to investigate since the dynamics of the original system, when restricted to the manifold, reduce to a lower dimensional set of equations (the inertial form). The inertial manifold is, therefore, a useful notion in the field of mechanism reduction. 

The combustion process of wet wood chips and formation of pollutants in a biomass furnace have been investigated. Distributions of species CO, UHC, O2 where calculated numerically and compared to experimental data. It is shown that char, as flying particle, though in small amount has a significant influence on the CO emissions at the outlet. Numerical simulation indicates that half of the CO emission at the outlet is due to the combustion of flying char particles at the upper part of the furnace. Over-fire air staging has a significant influence on the residence time of panicles and gas species in the furnace, and thus the conversion of fuel and intermediate species to final products. 

In reality, this very complicated process consists of hundreds of reactions. The hydrocarbon radicals are intermediate species formed during the combustion process. Prompt NOx is generally an important mechanism at lower-temperature combustion processes. Fuel NOx is formed by the direct oxidation of organo-nitrogen compounds contained in the fuel. It is given by the overall reaction ... 

Bockhom, H., Fetting, F., and Wenz, H. W. (1983) Chemistry of intermediate species in rich combustion of benzene, in Soot Formation in Combustion Systems and Its Toxic Properties, J. Lahaye, ed., Plenum Press, New York, pp. 57-94. 

The hypothesis that 1,3-butadiene may be responsible for benzene production is inconsistent with the data presented here. However, this conclusion is based on the low measured mole fractions of the intermediate species cyclohexadiene (C Hg) which may be peculiar to these experimental conditions. Therefore, this conclusion does not rule out the possible significance of 1,3-butadiene in other combustion systems. 

One of the most relevant works published recently in that field is concerned with intermediate species produced during combustion of benzene (17)- We begin to examine PAH formation from aromatic fuels. 

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