Radical pool

The essential feature of the initiation step is to provide a radical for the chain system and, as discussed in the previous section, the actual initiation step is not important in determining the explosive condition, nor is it important in determining the products formed. Either reaction (3.14) or (3.16) provides an H radical that develops a radical pool of OH, O, and H by the chain reactions... 

The sequence [Eqs. (17)—(20)] is of great importance in the oxidation reaction mechanisms of any hydrocarbon in that it provides the essential chain branching and propagating steps as well as the radical pool for fast reaction. 

The H atom introduced by reaction (3.66) and the OH, which arises from the H02, initiate the H radical pool that comes about from reactions (3.17)— (3.20). The subsequent decay of the aldehyde is then given by... 

Once the radical pool forms, the disappearance of the fuel is controlled by the reactions... 

The H2—02 radical pool that then develops begins the reactions that cause the fuel concentration to decay. The most effective attackers of the methyl side chain of toluene are OH and H. OH does not add to the ring, but rather abstracts a H from the methyl side chain. This side-chain H is called a benzylic H. The attacking H has been found not only to abstract the benzylic H, but also to displace the methyl group to form benzene and a methyl radical [69], The reactions are then... 

The first step of other high-order alkylated aromatics proceeds through pyrolytic cleavage of a CC bond. The radicals formed soon decay to give H atoms that initiate the H2—02 radical pool. The decay of the initial fuel is dominated by radical attack by OH and H, or possibly O and H02, which abstract an H from the side chain. The benzylic H atoms (those attached to the carbon next to the ring) are somewhat easier to remove because of their lower... 

As is undoubtedly apparent, the kinetic route of NO formation is not the attack of an oxygen molecule on a nitrogen molecule. Mechanistically, as described in Chapter 3, oxygen atoms form from the H2—02 radical pool, or possibly from the dissociation of 02, and these oxygen atoms attack nitrogen molecules to start the simple chain shown by reactions (8.49) and (8.50) ... 

In a large radical pool, there exists an equilibrium,... 

Following the conceptual idea introduced by Milliken [68], Takahashi and Glassman [53] have shown, with appropriate assumptions, that, at a fixed temperature, i/c could correlate with the number of C—C bonds in the fuel and that a plot of the log ipc versus number of C—C bonds should give a straight line. This parameter, number of C—C bonds, serves as a measure of both the size of the fuel molecule and the C/H ratio. In pyrolysis, since the activation energies of hydrocarbon fuels vary only slightly, molecular size increases the radical pool size. This increase can be regarded as an increase in the Arrhenius pre-exponential factor for the overall rate coefficient and hence in the pyrolysis and precursor formation rates so that the C/H ratio determines the OH concentration [12]. The 4>c versus C—C bond plot is shown in Fig. 8.14. When these... 

The initiation reactions such as (R6) serve to build up the radical concentration in a chain reaction only these steps may add to the number of radicals in the system. The propagating steps do not affect the overall radical concentration. The time to develop a radical pool sufficient for onset of fast reaction is called the induction time. 

In combustion systems it is generally desirable to minimize the concentration of intermediates, since it is important to obtain complete oxidation of the fuel. Figure 13.5 shows modeling predictions for oxidation of methane in a batch reactor maintained at constant temperature and pressure. After an induction time the rate of CH4 consumption increases as a radical pool develops. The formaldehyde intermediate builds up at reaction times below 100 ms, but then reaches a pseudo-steady state, where CH2O formed is rapidly oxidized further to CO. Carbon monoxide oxidation is slow as long as CH4 is still present in the reaction system once CH4 is depleted, CO (and the remaining CH2O) is rapidly oxidized to CO2. 

The methoxy radical may subsequently react to form formaldehyde (H atom abstraction) or methanol (H atom addition). The sequence of reactions (R15) through (R17) is strongly chain branching and serves to build up a radical pool. Once this radical pool is established, another chain-branching oxidation route becomes dominating. Methane consumption now occurs mainly by the reactions [254]... 

Methanol is a desired end product in partial methane oxidation, but some methanol is consumed by reaction with the radical pool,... 

After buildup of the radical pool, CH3 is mainly consumed by reaction with O atoms,... 

Even though this is a chain-terminating step, the radical pool is rapidly replenished through the H + O2 reaction (Rl). Reactions between formaldehyde and O/H radicals lead to the formyl radical (HCO), which subsequently dissociates thermally (R30) or reacts with O2 to form CO (R31). 

Reactions of propene (C3H6) with the radical pool may lead to formation of allyl (Q3H5) isomers (CH2CHCH2, CH2CCH3, CHCHCH3),... 

Other consumption steps for C6H5 include thermal dissociation, interaction with the radical pool, and reactions with aromatic and linear hydrocarbons. 

In combustion, we have seen that fuel-sulfur is rapidly oxidized to sulfur oxides, primarily SO2. Sulfur dioxide may subsequently interact with the O-H radical pool in the postflame region. This interaction is important for two reasons. Sulfur dioxide is known to catalyze the recombination of the main chain carriers in the flame through the sequence... 

The SO2 + O + M reaction (R145), followed by partial recycling of SO3 to SO2 by reaction with the radical pool, is the main radical sink under lean conditions. Because of the low thermal stability of HOSO2, the importance of the SO2 + OH + M recombination reaction (R145) is limited at combustion conditions. 

Hydrogen chloride typically is the desired chlorine-containing product in combustion, because it can easily be removed from the flue gas by a scrubbing process. However, even HCI may create some problems, because it can inhibit oxidation of combustibles such as CO under postflame conditions. The interaction of HCI with the O-H radical pool is quite complex, and even though the overall mechanism of inhibition is known [336], details are still under investigation. 

The first step in the inhibition is reaction of hydrogen chloride with the radical pool, mainly... 

Almost all of the reactions of radicals can be grouped into three classes redox reactions, atom (or group) transfer reactions and addition reactions. A detailed discussion of these reactions is beyond the scope of this chapter, but a summary of some important features (with references to more in-depth discussions) is essential. Although addition reactions will receive the most attention, redox and atom transfer reactions are important because nearly all radicals formed by addition reactions will be removed from the radical pool to give nonradical products by one of these methods. 

---