Inhibition of explosion limits by hydrocarbons

All the simple hydrocarbons are able to suppress the low pressure ignition of the H2 + O2 system. However, there are major differences of behaviour between methane and neopentane on the one hand, and most other hydrocarbons and related materials on the other [329—332]. With formaldehyde [333], ethane [334—336], propane [329, 337], and n- and i-butane [338] the second limit in KCl coated vessels falls more or less linearly with increasing partial pressure of additive. In the experiments of Baldwin et al. [333—338], the mole fractions, x and y, of H2 and O2, respectively, could be varied independently of each other by working with H2 + N2 + O2 mixtures and adjusting the nitrogen content appropriately. The rate of fall of the second limit at constant x was almost inversely proportional to y while at constant y and not too small x, it was almost independent of x. The limit did not change much with vessel size. The observations may be accounted for by adding reactions (1)—(lii) 

It appears that the alkyl radicals formed react predominantly with O2 to form HO2 and an olefin (or CO in the case of formaldehyde). HO2 formation at second limit pressures in a KCl coated vessel is essentially a chain terminating step (cf. Sect. 3.6). If t i/2 denotes the inhibitor concentration required to halve the second limit, then I1/2 is inversely proportional to the rate of fall of the limit, and the scheme leads to 

Thus the observed marked dependence of ij j2 ony indicates uniquely the importance of reaction (li). The small variation of ii/2 with x, observed principally at low x, is associated with contributions from reactions (1) and (lii). These are difficult to separate, but their inclusion under the 

Ratios of rate coefficients for hydrocarbon inhibition of second limits [70] 

Of the alkyl silanes, tetraethylsilane behaves like ethane, propane and butane, but tetramethylsilane behaves like methane [343]. 

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