Hydrocarbons slow combustion

Oxidation of Hydrocarbons. Ethanol is one of a variety of oxygen-containing compounds produced by the oxidation of hydrocarbons. Ethanol is reported to be obtained in a yield of 51% by the slow combustion of ethane (158,159). When propane is oxidi2ed at 350°C under a pressure of 17.2 MPa (170 atm) (160,161), 8% of the oxygen is converted to ethanol. Lower conversions to ethanol are obtained by oxidi2ing butane. Other oxidation systems used to produce ethanol and acetaldehyde (162—164) and methods for separating the products have been described in the patent Hterature. 

The slow combustion reactions of acetone, methyl ethyl ketone, and diethyl ketone possess most of the features of hydrocarbon oxidation, but their mechanisms are simpler since the confusing effects of olefin formation are unimportant. Specifically, the low temperature combustion of acetone is simpler than that of propane, and the intermediate responsible for degenerate chain branching is methyl hydroperoxide. The Arrhenius parameters for its unimolecular decomposition can be derived by the theory previously developed by Knox. Analytical studies of the slow combustion of methyl ethyl ketone and diethyl ketone show many similarities to that of acetone. The reactions of methyl radicals with oxygen are considered in relation to their thermochemistry. Competition between them provides a simple explanation of the negative temperature coefficient and of cool flames. 

It was the late Professor Callender (12), who as an outcome of experiments upon the slow combustion of hexane which resulted in the formation of valeraldenyde, acetaldehyde, and fonnaldehyde without any detectable initial hexyl alcohol, CeHi3OH, first suggested that the initial oxidation of a hydrocarbon in air more probably involves the formation of an alkyl peroxide by the direct incorporation of the oxygen molecule in the hydrocarbon molecule after the collision which subsequently decomposes into aldehydes and water thus... 

It has been suggested that, just as in the combustion of carbon and the slow combustion of hydrocarbon gases, the first step in the slow combustion of coal consists in the formation of an additive compound or complex, consisting of oxygen and one or more of the substances present in coal.10... 

The cool flame phenomenon seems to be closely tied to the formation of aldehydes and peroxides in oxidation systems. In Fig. XIV. 10 is shown a typical example of the explosion limits for a hydrocarbon-oxygen mixture. The explosion region, except for a region of positive slope, resembles the limit curve for a thermal explosion. The transition between slow combustion and explosion is characterized by an intense luminous blue flame which appears after a short induction period and is followed by explosion. The induction periods are not more than a few seconds. 

The slow combustion of the lower aliphatic amines occurs at temperatures and pressures comparable with those under which the corresponding hydrocarbons also react. 

The slow combustion of methylene chloride is a degenerately branched chain reaction it proceeds by a mechanism similar to that involved in the pyrolysis of the same compound which takes place at a slightly higher temperature [153]. The primary chains are the same and several of the chlorinated hydrocarbon minor products are identical. Oxygen is only involved in the conversion of the intermediate dichloroethylene to the final products hydrogen chloride and carbon monoxide. 

Notwithstanding the obstacles, however, some absorption studies of combustion processes have been made. Molecular intermediates, such as aldehydes and acids, have been identified in the slow combustion of propane . Hydroxyl radicals can be observed in the absorption spectra of several flames . The greatest success in the application of absorption spectroscopy to flame studies has been in investigations of diffusion flames. Wolfhard and Parker studied the diffusion flames in oxygen of hydrogen, ammonia, hydrocarbons and carbon monoxide. In every case they were able to observe absorption by hydroxyl radicals, and they observed also the absorption of NH in the ammonia flame (NH2 appeared in emission only). Molecular oxygen, and in suitable cases the reactants, could be detected by their absorption spectra, so that a clear picture of the structure of the diffusion flame... 

Saturated hydrocarbons are inert to being absorbed therefore, they are determined by combustion with O2. Figure 42-1 shows an uncovered slow combustion pipet, which also is shown in Figure 42-3, p. 495, on the left side. Mercury must be used as the retaining liquid. A measured volume of the gas or mixture of gases is combusted in a measured volume of Oj. The volume contraction, the amount of O, consumed, and the amount of CO, produced are measured. From these three pieces of data, you can calculate three unknown quantities. You can do this ... 

From the standpoint of atmospheric pollution, hydrocarbon combustion is of major importance. Depending on conditions of the combustion processes, the balance of the proportions of final products may be considerable different. Particular differences are observed for mixtures containing different amounts of air. The composition of products changes also with temperature. At low temperatures (700 K) a slow combustion occurs, at temperatures from 700 to 1500 K, the process of the hydrocarbon oxidation is essentially more rapid with a tendency to explosion. In a comparison with the moderate combustion, the mechanism of the process is different. At very high temperatures (> 2300 K) an extremely rapid, explosive combustion is observed. 

In a spatial structure such as a hard foam with quick-burning mixtures (H2+ air or light hydrocarbon mixtures like propane + oxygen -I- some nitrogen), fast quasidetonation processes have developed. When some hydrocarbon + air mixtures were used, flame quenching was observed [24]. No slow combustion regimes were reported [24]. 

Fig. 6. Schematic ignition diagram for a hydrocarbon+ O2 mixture, with appHcations. Region A, very rapid combustion, eg, a jet engine region B, low temperature ignition, eg, internal combustion engine, safety ha2ards regions C and D, slow oxidation to useful chemicals, eg, 0-heterocycHc compounds in C and alcohols and peroxides in D. Courtesy of Blackwell Scientific PubHcations, Ltd., Oxford (60).

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Chemical explosions are uniform or propagating explosions. An explosion in a vessel tends to be a uniform explosion, while an explosion in a long pipe is a propagating explosion. Explosions are deflagrations or detonations. In a deflagration, the burn is relatively slow, for hydrocarbon air mixtures the deflagration velocity is of the order of 1 m/s. In contrast, a detonation flame shock front is followed closely by a combustion wave that releases energy to sustain the shock wave. A... 

For a chemical reaction such as combustion to proceed, mixing of the reactants on a molecular scale is necessary. However, molecular diffusion is a very slow process. Dilution of a 10-m diameter sphere of pure hydrocarbons, for instance, down to a flammable composition in its center by molecular diffusion alone takes more than a year. On the other hand, only a few seconds are required for a similar dilution by molecular diffusion of a 1-cm sphere. Thus, dilution by molecular diffusion is most effective on small-scale fluctuations in the composition. These fluctuations are continuously generated by turbulent convective motion. 

Gas turbines and power stations are particularly prone to generate NOx and the search for the low-NOx burner that will operate at high efficiency (i.e. with low hydrocarbon emissions) continues. The principle of the low-NOx burner is to slow the rate of combustion by dividing it into several stages by the gradual mixing of the combustion gases with the stoichiometric air volume. 

Diffusion Flame. When a slow stream of fuel g s flows from a tube into the atmosphere, air diffuses across the boundary of the stream and Brms an envelope of expl mixture around a core of gas. The core decreases in height until it disappears at some distance above the tube. It thus assumes the shape of a cone. On ignition, a flame front spreads thru the mixture and stabilizes itself around the cooe of fuel gas. The hydrocarbons in common fuel gases crack to form free C H. The shell of carbon-bearing gas so formed gives such flames their luminosity Turbulent Jet Flame. When a gas stream issues from an orifice above a certain critical velocity, it breaks up into a turbulent jet that entrains the surrounding air. The flame of such a jet consists of random patches of combustion and no cohesive combustion surface exists... 

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