Combustion elementary reactions

Volume 16 Volume 1 7 Liquid-phase Oxidation Gas-phase Combustion Section 7. SELECTED ELEMENTARY REACTIONS (1 volume)... 

Our treatment of chain reactions has been confined to relatively simple situations where the number of participating species and their possible reactions have been sharply bounded. Most free-radical reactions of industrial importance involve many more species. The set of possible reactions is unbounded in polymerizations, and it is perhaps bounded but very large in processes such as naptha cracking and combustion. Perhaps the elementary reactions can be postulated, but the rate constants are generally unknown. The quasi-steady hypothesis provides a functional form for the rate equations that can be used to fit experimental data. 

Developments in computer techniques making it possible to solve complicated fluid motions in a combustion environment that are affected by diffusion and involve complicated chemistry (large numbers of elementary reactions, which individually are not "complex" but quite simple, i.e., most of them involve two reacting species, sometimes three, and the formation or breaking of just one bond), and with a large number of transient intermediates formed in the course of fuel oxidation and pollutant formation. 

Miller, J.A., Pilling, M.J., and Troe, J., Unravelling combustion mechanisms through a quantitative understanding of elementary reactions, Proc. Combust. Inst., 30,43,2005. 

An elementary step must necessarily be simple. The reactants are together with sufficient energy for a very short time, and only simple rearrangements can be accomplished. In addition, complex rearrangements tend to require more energy. Thus, almost all elementary steps break and/or make one or two bonds. In the combustion of methane, the following steps (among many others) occur as elementary reactions ... 

Section 6. oxidation and combustion reactions (2 volumes) Section 7. selected elementary reactions (2 volumes)... 

A further interaction comes into play when the thermal DeNOx process is used to reduce NO,.. When stack gases cool and initial sulfur is present in the fuel, the S03 that forms reacts with water to form a mist of sulfuric acid, which is detrimental to the physical plant. Furthermore, the ammonia from the thermal DeNO, process reacts with water to form NH4HS02—a glue-like, highly corrosive compound. These S03 conditions can be avoided by reducing the S03 back to S02. Under stack (post-combustion) temperatures, the principal elementary reactions for S03 to S02 conversion are... 

However, the combustion process for methane requires no fewer than 325 individual mechanistic steps (elementary reactions) to be accurately described, rather than the one-step route shown above. As such, incomplete combustion is a common occurrence and ROS are pervasive byproducts of that phenomenon, affecting an engine s fuel efficiency and producing atmospherically detrimental emissions. Moreover, combustion varies with system temperature, as different oxidative pathways become accessible, as well as fuel/oxidizer ratio (equivalence ratio). By examining the representative cases of methane oxidation at high and low temperatures, this phenomenon becomes clearer. 

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. 

Common quantum mechanical methods for exploring the energetics of elementary reaction steps include ab initio and density functional theory (DFT). " As computational speeds have increased, use of higher levels of theory have allowed for more accurate prediction of properties and reactions for reactive radical intermediates, further advancing our understanding of combustion chemistry. 

Using these methods, the elementary reaction steps that define a fuel s overall combustion can be compiled, generating an overall combustion mechanism. Combustion simulation software, like CHEMKIN, takes as input a fuel s combustion mechanism and other system parameters, along with a reactor model, and simulates a complex combustion environment (Fig. 4). For instance, one of CHEMKIN s applications can simulate the behavior of a flame in a given fuel, providing a wealth of information about flame speed, key intermediates, and dominant reactions. Computational fluid dynamics can be combined with detailed chemical kinetic models to also be able to simulate turbulent flames and macroscopic combustion environments. 

Notably, the Gas Research Institute s mechanism (GRI-MECH) for methane combustion is well-established, drawing on research from several groups over several decades to define and calibrate kinetic and thermodynamic data for each elementary reaction step. Additional mechanisms" for methane oxidation are also available and updated periodically to include the most recent data. 

Thermal reactions of light alkanes with oxygen in the combustion process have been studied extensively (6, 7). These studies were typically conducted at high temperatures—flame temperatures. The elementary reactions of the hydrocarbon species often involve reactions with atomic (H, O) or free radical species (OH, alkyl, etc.). The initiation step is the homolytic cleavage of C—C single bonds to form alkyl radicals. The C—C bonds are the weakest bonds in an alkane molecule (Table I). The chain-propagation step... 

In many cases the goal is to understand the observed reaction rate behavior on a more fundamental basis. An observed, overall reaction can be the net result of a number of simpler elementary reaction steps. For example, the overall reaction for the complete combustion of methane is... 

However, the actual combustion process takes place by a manifold of hundreds of elementary reactions. Thus the kinetics exhibit complex behavior as a function of temperature, pressure, flow rates, and so on. 

A number of printed data evaluations for elementary reactions are available. These include compilations that specifically address combustion chemistry ... 

Reactions (Rl) and (R12) are the two most important elementary reactions in combustion. H + O2 is the essential chain-branching reaction, while CO + OH is a chain-propagating step that regenerates the H atom from OH. Furthermore the CO + OH reaction is highly exothermic and responsible for a large fraction of the heat release that occurs in combustion of hydrocarbon fuels. Under moist conditions, reactions of CO with O and O2 are not competitive, but (RIO) may serve as an initiation step. 

From the technology of combustion we move to the molecular mechanism of flame propagation. We shall give a molecular-kinetic expression for the heat release rate by calculating the frequency v of collisions of fuel molecules with other molecules (v is proportional to the molecular velocity and inversely proportional to the mean free path), further taking into account that only a small (1/j/) part of all collisions are effective. The quantity 1/v—the probability of reaction taken with respect to a single collision— depends on the activation heat of an elementary reaction event, as well as on the fraction of all molecules comprised of those radicals or atoms by means of which the reaction occurs. The molecular-kinetic expression for the coefficient of thermal conductivity follows from formulas (1.2.4) and (1.2.3). 

Similar reaction sequences have been identified in other chemically reacting systems, specifically catalytic combustion (52, 53), solid-fuel combustion (54), transport and reaction in high-temperature incandescent lamps (55), and heterogeneous catalysis (56 and references within). The elementary reactions in hydrocarbon combustion are better understood than most CVD gas-phase reactions are. Similarly, the surface reaction mechanisms underlying hydrocarbon catalysis are better known than CVD surface reactions. 

Such questions are answered empirically all too often. A more fundamental approach is needed. In the area of gas-phase kinetics, the developments in the chemistry of large sets of elementary reactions and diffusion in multi-component mixtures in a combustion context are now finding applications in chemical engineering, as mentioned above. In the area of gas-solid reactions, the information flow will be in the opposite direction. A need exists... 

Which experimental approach can best reveal the chemical dynamics of carbon-centered radicals Recall that since the macroscopic alteration of combustion flames, atmospheres of planets and their moons, as well as of the interstellar medium consists of multiple elementary reactions that are a series of bimolecular encounters, a detailed understanding of the mechanisms involved at the most fundamental microscopic level is crucial. These are experiments under single collision conditions, in which particles of one supersonic beam are made to collide only with particles of a second beam. The crossed molecular beam technique represents the most versatile approach in the elucidation of the energetics and... 

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