Radicals combustion

The development of an ethane combustion mechanism provides a historical context for understanding some overall trends of alkyl radical combustion. An understanding of the likely pathways for this small system is useful in modeling chemistry of larger systems, as can be observed from an examination of some other reactive radical intermediates. 

This is a chain reaction with many similarities to the chain reactions that occur in free-radical combustion processes in the previous chapter. For the chain reaction A B + C we represented the process as a kinetic chain involving the chain-propagating intermediate R, which was fed and terminated by initiation and termination reactions (Figure 1 1-2). In the preceding reaction sequence AMj is involved in a similar chain, but now each time the chain goes around the molecule is increased in size by one monomer unit We can represent this process in Figure 11-3. This reaction system is a series reaction in AMj,... 

Flame-retardant styrenic polymers find utility in applications such as building insulation (expanded polystyrene foam) and electronic enclosures (flame-retardant HIPS, ABS and styrenic blends). The most effective flame retardants are halogen-(particularly bromine)-containing compounds these flame retardants act by inhibiting the radical combustion reactions occurring in the vapor phase. Flame-retardant plastics are in a state of flux, due to influences of... 

Glanzer K, Quack M and Troe J 1977 High temperature UV absorption and recombination of methyl radicals in shock waves Proc. 16th int. Symp. on Combustion (Pittsburg, PA The Combustion Institute) pp 949-60... 

Chemical Interaction. Halogens and some phosphoms flame retardants act by chemical interaction. The flame retardant dissociates into radical species that compete with chain propagating and branching steps in the combustion process. 

Phosphoms-containing additives can act in some cases by catalyzing thermal breakdown of the polymer melt, reducing viscosity and favoring the flow or drip of molten polymer from the combustion zone (25). On the other hand, red phosphoms [7723-14-0] has been shown to retard the nonoxidative pyrolysis of polyethylene (a radical scission). For that reason, the scavenging of radicals in the condensed phase has been proposed as one of several modes of action of red phosphoms (26). 

At combustion temperatures, the oxidation of butane [106-97-8] is similar to that of propane (153). This is because most butyl radicals are consumed by carbon—carbon bond scission (reaction 28). 

The alkanes have low reactivities as compared to other hydrocarbons. Much alkane chemistry involves free-radical chain reactions that occur under vigorous conditions, eg, combustion and pyrolysis. Isobutane exhibits a different chemical behavior than / -butane, owing in part to the presence of a tertiary carbon atom and to the stability of the associated free radical. 

Flame Retardants. Flame retardants are added to nylon to eliminate burning drips and to obtain short self-extinguishing times. Halogenated organics, together with catalysts such as antimony trioxide, are commonly used to give free-radical suppression in the vapor phase, thus inhibiting the combustion process. Some common additives are decabromodiphenyl oxide, brominated polystyrene, and chlorinated... 

When the partial pressures of the radicals become high, their homogeneous recombination reactions become fast, the heat evolution exceeds heat losses, and the temperature rise accelerates the consumption of any remaining fuel to produce more radicals. Around the maximum temperature, recombination reactions exhaust the radical supply and the heat evolution rate may not compensate for radiation losses. Thus the final approach to thermodynamic equiUbrium by recombination of OH, H, and O, at concentrations still many times the equiUbrium value, is often observed to occur over many milliseconds after the maximum temperature is attained, especially in the products of combustion at relatively low (<2000 K) temperatures. 

The radicals and other reaction components are related by various equiUbria, and hence their decay by recombination reactions occurs in essence as one process on which the complete conversion of CO to CO2 depends. Therefore, the hot products of combustion of any lean hydrocarbon flame typically have a higher CO content than the equiUbrium value, slowly decreasing toward the equiUbrium concentration (CO afterburning) along with the radicals, so that the oxidation of CO is actually a radical recombination process. 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

Free radicals are molecular fragments having one or more unpaired electrons, usually short-lived (milhseconds) and highly reaclive. They are detectable spectroscopically and some have been isolated. They occur as initiators and intermediates in such basic phenomena as oxidation, combustion, photolysis, and polvmerization. The rate equation of a process in which they are involved is developed on the postulate that each free radical is at equihbrium or its net rate of formation is zero. Several examples of free radical and catalytic mechanisms will be cited, aU possessing nonintegral power law or hyperbohc rate equations. 

Confirmation of the formation of the radicals during combustion reactions has been made by inuoducing a sample of dre flames into a mass spectrometer. The sample is withdrawn from a turbulent flame which is formed into a thin column, by admitting a sample of the flame to the spectrometer drrough a piidrole orifice, usually of diameter a few tenths of a millimetre. An alternative procedure which has been successful in identifying the presence of radicals, such as CHO, has been the use of laser-induced fluorescence. 

The individual steps in chain reactions involving radicals are characteristically of small activation energy, between about 10 and 50kJmol and so these reactions should occur at an immeasurably high rate at temperatures above 500 K (see Table 2.1), which is a low temperature for a useful combustion process. The overall rate of the process will tlrerefore depend mainly on the concentrations of tire radicals. 

Show a free radical reaction which results in ethane in the effluent of a combustion process burning pure methane with pure oxygen. 

Primary chemical processes. The external heat source may supply free radicals which accelerate combustion. The heating material might also be activated by autocatalytic or autoignition mechanisms. 

Some inorganic fillers are used as flame retardants in rubber base formulations. Flame retardants act in two ways (1) limiting or reducing access of oxygen to the combustion zone (2) reacting with free radicals (especially HO ), thus acting as terminator for combustion-propagation reaction. The additives most widely used as flame retardants for polymers are antimony oxides and alumina trihydrate. 

Whenever a hydrocarbon backbone has two hydroxyl radicals attached to it, it becomes a special type of alcohol known as a glycol. The simplest of the glycols, and the most important, is ethylene glycol, whose molecular formula C2H4(OH)2. The molecular formula can also be written CHjOHCHjOH and may be printed as such on some labels. Ethylene glycol is a colorless, thick liquid with a sweet taste, is toxic by ingestion and by inhalation, and among its many uses is a permanent antifreeze and coolant for automobiles. It is a combustible liquid with a flash point of 240"F. 

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