Reaction mechanisms pyrolysis

Species, 98 Reaction Pyrolysis Mechanism SCHEME II, CRAY-1, Optimized... 

In a related reaction, pyrolysis of allylic ethers that contain at least one a hydrogen gives alkenes and aldehydes or ketones. The mechanism is also pericyclic"" ... 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

The thermal decomposition and photolysis of this alkyl have been studied by Buchanan and Creutzberg112. The pyrolysis mechanism is not fully understood. The overall process is first-order and is unaffected by an 8.5-fold increase in surface-to-volume ratio. Based on measurements of pressure increase, the reaction exhibits an induction period ranging from 2-3 minutes at 513 °C to 40 minutes at 466 °C. Short chains are apparently involved. A polymer initially of empirical formula (BCH2) but slowly losing hydrogen to form (BCH) is deposited on the surface. The mechanism probably involves the reactions... 

A CH4 pyrolysis mechanism appears to be consistent with our observation that preheating improves partial oxidation selectivity. First, higher feed temperatures increase the adiabatic surface temperature and consequently decrease the surface coverage of O adatoms, thus decreasing reactions lOa-d. Second, high surface temperatures also increase the rate of H atom recombination and desorption of H2, reaction 9b. Third, methane adsorption on Pt and Rh is known to be an activated process. From molecular beam experiments which examined methane chemisorption on Pt and Rh (79-27), it is known that CH4 must overcome an activation energy barrier for chemisorption to occur. Thus, the rate of reaction 9a is accelerated exponentially by hi er temperatures, which is consistent with the data in Figure 1. 

This unexpected result may be related to the increase in TOC on fraction G3 and may be further evidence of the polymerization phenomenon discussed earlier. However, this hypothesis must be carefully considered because of our limited knowledge of pyrolysis mechanisms. The possibility of phenol formation during the thermal fragmentation process from elimination reactions followed by cycliza-tion of poly conjugated chains has been suggested by Bracewell (22) and should be investigated. 

Even though free-radical reaction schemes best represent the pyrolysis mechanisms, simple nth-order models for the pyrolysis of the lighter feedstocks have been proposed. The range of orders is from 0.5 to 2.0, and significant differences are often reported in the literature. The overall activation energies vary from about 50 to 95 kcal/g mol. 

Mechanistic modeling has been useful in studying pyrolysis kinetics at low conversion (4,5,6). Few attempts have been reported at the high conversion levels of commercial cracking (7). This stems from the large number of species and free radicals and of their associated reactions, which increases substantially with conversion and leads to excessive computation time. In addition, when one considers that precise pyrolysis mechanisms, for even a simple feedstock such as propane (8), are still subject to dispute, it is clear that more empirical models will continue to dominate commercial applications. 

Thanks to this work, and subsequent work on pyrolysis mechanisms, it has become clear that three different types of elementary reactions are involved (see Chapter 2, Volume 2 for a discussion of chain reactions). 

Mechanism. No single mechanism explains the action of all fire retardants, so they probably work through a combination of several mechanisms. The mechanisms of fire retardants in wood involve a complex series of simultaneous reactions whose products may affect subsequent reactions. Pyrolysis of cellulose involves dehydration, depolymerization, decarbonylation, decomposition of smaller compounds, condensation, and other reactions. These pyrolysis reactions occur both in the solid phase and vapor phase. Addition of fire retardants will alter the reactions however, this alteration will depend on the additives, the material, and the thermal-physical environment. The presence of oxygen adds subsequent and competitive oxidation reactions to the above series. These oxidative reactions can take place in both the solid and vapor phases. Evidence indicates that most fire retardants reduce combustible volatiles production and limit combustion to the solid phase. The best retardants also inhibit solid-phase oxidation to effectively remove the fuel from the fire. 

The chain scission can be seen as a pyrolytic elimination reaction. All mechanisms described in Section 2.2 may take place during chain scission. A reaction of chain scission with a cyclic transition state may take place, for example, during cellulose pyrolysis ... 

The decomposition process seems to start with three competing reactions followed by further decompositions. The main initial pyrolysis mechanisms probably are... 

The differences between the copolymers with different microstructures is, however, not always straightforward, and depends on the pyrolysis mechanism. Polymers undergoing side chain reactions may generate a more complex picture for the pyrogram and the quantitative evaluation of block or random character can be more difficult. 

Because char commands a low market value, there is some incentive to increase gas production at the expense of char formation. The traditional approach is to use the water gas, Boudouard, and combustion reactions (step 3) to gasify the char produced by step 1. An alternative approach is to rapidly heat solid biomass feed, modifying the pyrolysis mechanism (step 1) and reducing the initial formation of char by the pyrolysis reactions. The latter approach has been emphasized in the research reported here. 

The K-butane pyrolysis is analysed here as an initial, simple example of a pyrolysis reaction mechanism. It is important to note that the pyrolysis reactions of small hydrocarbons are fundamental to the proper understanding of the whole process. In fact, the pyrolysis mechanism displays a typical hierarchical structure and the small hydrocarbons must be analysed first. Fig. 1 shows the main and minor products from K-butane decomposition, under isothermal conditions, at 1,093 K and 1 atm. Ethylene, propylene and methane are the main products, while only trace amount of butenes, ethane, benzene and cyclopenta-diene are observed. These model predictions have been confirmed and validated by several experimental measurements (Dente and Ranzi, 1983). 

The simple pyrolysis of n-butane gives only a partial idea of how complex pyrolysis mechanisms are. The pyrolysis of n-decane provides a further example of the complexity of pyrolysis reactions. 

As an example of application of automatic generation, Table I gives the complete set of the primary propagation reactions of -decyl radicals isomerization, -decomposition and dehydrogenation reactions. These reactions are produced directly by the MAMA program which was specifically developed for pyrolysis mechanism generation (Dente and Ranzi, 1983 Dente et al., 2005 Pierucci et al., 2005). 

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