The Combustion of Hydrocarbons

From the point of view of their technological and economical position in our industrialized society, the oxidation reactions of the hydrocarbons are probably of unrivaled importance. The utilization of partial oxidation reactions in the synthesis of important industrial compounds, while a poor second to the role of oxidation for production of energy, is nevertheless a colossal and growing field. Parallel to the utility of these reactions is their chemical complexity and the multitude of unique phenomena associated with their kinetic behavior. 

At relatively low temperature (for example, 100 to 200 C), the gas-phase oxidation of aliphatic hydrocarbons in the absence of catalysts is immeas- 

In the neighborhood of 300 to 400°C, the aliphatic hydrocarbons from ethane on up show a rather complex ignition behavior (Fig. XIV. 10). At 

Although there is no direct information on the method of initiation of radicals in the combustion reactions, there seems to be fairly general agreement on the reaction 

At low temperatures fairly long chains leading to the production of hydroperoxides can be set up  

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H. C. Barnett and R. R. Hibbard, Basic Considerations in the Combustion of Hydrocarbon Fuels with Air, NASA Technical Report, 1959, p. 1300. 

Catalyst Function. Automobile exhaust catalysts are perfect examples of materials that accelerate a chemical reaction but are not consumed. Reactions are completed on the catalyst surface and the products leave. Thus the catalyst performs its function over and over again. The catalyst also permits reactions to occur at considerably lower temperatures. For instance, CO reacts with oxygen above 700°C at a substantial rate. An automobile exhaust catalyst enables the reaction to occur at a temperature of about 250°C and at a much faster rate and in a smaller reactor volume. This is also the case for the combustion of hydrocarbons. 

Carbon dioxide has been implicated as a contributing factor in global warming. Increased global warming has been associated with increased release of carbon dioxide into the atmosphere attributed in part to an increase in the combustion of hydrocarbon fuels. Carbon dioxide is an inevitable consequence of the complete combustion of hydrocarbons in air. If combustion devices are made more efficient, less fuel is required and less carbon dioxide is released into the atmosphere. 

F.19 The CO, produced by the combustion of hydrocarbons contributes to global warming. Rank the following fuels according to increasing mass percentage of carbon (a) ethene, C2H4 (b) propanol, C3H-OI I (c) heptane, C-Hl6. 

Methyl and ethyl nitrates are promoters of the combustion of hydrocarbons. Nitrobenzene and aromatic nitrated explosives are mostly insensitive to impact. They become sensitive when potassium or Na-K alloy is present. 4-Nitrotoluene with sodium in diethyl ether medium forms a black residue that combusts spontaneously in air. 

In the next investigation, you will carry out several redox reactions, including reactions of acids with metals, and the combustion of hydrocarbons. 

The chosen combinations of these chemicals and metals depend on the requirements of the specific application. Gasless combustion prevents pressure increase in a closed combustion chamber. Some combinations of metal particles and metal oxide particles or of metal particles and crystalline oxidizers are chosen as chemical ingredients of gasless pyrolants. On the other hand, hydrocarbon polymers are used to obtain combustion products of low molecular mass, such as H2O, CO, CO2, and H2. High pressure is thus obtained by the combustion of hydrocarbon polymers. Table 10.6 shows the chemical ingredients used to formulate various types of pyrolants. 

An important feature of the high-temperature oxidation of hydrocarbons is the progressive accumulation of formaldehyde during the induction period and the influence that it may exert on the subsequent course of reaction. Detailed studies of the role of formaldehyde in the combustion of hydrocarbons at temperatures above 400° C. have been reported for methane, ethylene, and cyclopropane. 

What factors determine the direction and extent of a chemical reaction Some reactions, such as the combustion of hydrocarbon fuels, go almost to completion. Others, such as the combination of gold and oxygen, occur hardly at all. Still others—for example, the industrial synthesis of ammonia from N2 and H2 at 400-500°C— result in an equilibrium mixture that contains appreciable amounts of both reactants and products. 

Carbon dioxide can also serve as a convenient reagent in selected reduction processes, and interest in its use as an inexpensive source of chemical carbon has greatly intensified over the past few years (2, 3). Carbon dioxide is a very stable molecule in as much as it is the thermodynamic end product of many energy producing processes [e.g., the combustion of hydrocarbons, Eq. (1)]. Hence, its use will require a major input of energy. The reactivity of... 

With regard to the susceptibility of many organometallics to aerial oxidation, it must be remembered that most compounds containing C-H bonds are thermodynamically unstable with respect to oxidation by molecular oxygen to produce water and carbon dioxide. The mechanisms whereby (for example) many alkyls are spontaneously flammable in air are necessarily difficult to determine the mechanisms involved in the combustion of hydrocarbons are still incompletely understood. Suffice it to say that the kinetic stability of ER to air seems to be low where E is of low electronegativity. 

A flame ionization detector (FID) is a measurement device which is specific to hydrocarbons, with a measurement principle based on the combustion of hydrocarbons in a hydrogen flame. 

The primary pollution problem in nitric add manufacture is the abatement of nitrogen oxides (NOx) in tail gases. In the United States in 1998, gaseous emissions from newly constructed nitric add plants must be limited to 1.5 kilograms of NOx per tonne of nitric add (100% basis) produced, with a maximum stack opacity of 10%. Modern add towers, with extended sections, can reduce NOx emissions to less than 200 parts per million35. Production of ammonia also involves the combustion of hydrocarbons so NOx abatement must also be addressed for this product as well. 

Petrella, R. V. Studies of the combustion of hydrocarbons by kinetic spectroscopy. II. The explosive combustion of styrene inhibited by halogen compounds, private communication, 1972. 

Photochemical production of alkyl radicals at temperatures between room temperature and 200°C. should enable the oxidation of alkyl radicals to be studied without interference from the molecular products undergoing oxidation and without the occurrence of a chain reaction. The low temperature would inevitably mean that fewer reactions took place, but it might also mean that the alkyl radical oxidation no longer occurred by the same mechanism as in the combustion of hydrocarbons. 

These are reactions where oxygen reacts with another substance, often producing energy in the form of heat and light. These reactions typically involve hydrocarbons, which are compounds of hydrogen and carbon. When hydrocarbons react with oxygen, they generate water vapor and either carbon dioxide or carbon monoxide as products. For the combustion of hydrocarbons, there are two types complete and incomplete. 

Indeed, it receives strong support from the work of Bone on the combustion of hydrocarbon gases (see pp. 65-67). Although not definitely proved, this attractive theory is certainly a most suggestive one it not only fits m well with known facts, but is in harmony with the various points urged m favour of each of the twro earlier theories. 

In the next section, you will learn about an important factor of combustion reactions energy. The combustion of hydrocarbons produces a large amount of energy. This is why they are so useful as fuels. How can you include energy as part of a combustion or other equation How can you calculate the energy released by fossil fuels You will learn the answers to these questions in the rest of this chapter. 

Detailed kinetic mechanisms for the combustion of hydrocarbon fuels have been subjected to relatively intensive study in recent years. The mechanisms are complex and involve a variety of chain carriers. Fundamental to such studies are data on rates of elementary steps. Extensive compilations of rate information are becoming available [12], [39]-[47]. Users should realize that uncertainties remain in rates of various elementary steps. Since these uncertainties sometimes exceed an order of magnitude, studies of... 

Modern society probably depends more upon the combustion of hydrocarbons than upon any other chemical reaction. The various applications of hydrocarbon combustion were recently described [1] in terms of a typical temperature—pressure ignition diagram for a hydrocarbon + oxygen (or air) mixture, as shown in Fig. 1. 

It appeared that the transfer of heat back from the highly exothermic final stages to the earlier parts was crucial in establishing the stable flame. Most of the products could be accounted for by reactions of types frequently postulated in the combustion of hydrocarbons and their derivatives. Thus, for example, 2-methyl-l 3-dioxacyclopentane could arise by... 

Volume 17 covers gas-phase combustion, which includes probably the most complex processes investigated by chemists. Chapter 1, about half the book, deals with the oxidation of hydrogen and carbon monoxide, with extensive consideration of all the individual reactions occurring. In Chapter 2, the combustion of hydrocarbons is discussed, with emphasis on the general mechanisms which have been suggested to account for the numerous products of partial oxidation. In Chapter 3, the oxidation of aldehydes, which are important intermediates in combustion of other compounds, is considered, and in Chapter 4, the oxidation of alcohols, ketones, oxirans, ethers, esters, peroxides, amines and halocarbons. 

The use of catalytic converters to reduce the amount of unbumed hydrocarbons in exhaust gases is an additional example of the use of metals. Reactions of these unbumed hydrocarbons in the atmosphere are described later, in the section on photochemical smog. The catalyst currently used is a cordierite or alumina support treated with an AI2O3 wash coat containing rare earth oxides and 0.10% to 0.15% Pt, Pd, and/or Rh, which catalyzes the combustion of hydrocarbons in the exhaust gases to carbon dioxide and water. Platinum,... 

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