Flame carbon formation

The formation of carbon black in a candle flame was the subject of a series of lectures in the 1860s by Michael Faraday at the Royal Institution in London (23). Faraday described the nature of the diffusion flame, the products of combustion, the decomposition of the paraffin wax to form hydrogen and carbon, the luminosity of the flame because of incandescent carbon particles, and the destmctive oxidation of the carbon by the air surrounding the flame. Since Faraday s time, many theories have been proposed to account for carbon formation in a diffusion flame, but controversy still exists regarding the mechanism (24). 

In fact, such polymerization reactions may play an important role in carbon formation in flames (26). 

Laboratory and industrial-scale processes show that acetylene is one intermediate in carbon formation in the combustion of petroleum hydrocarbons. This is not only a result of thermal decomposition but a part of the complex of reactions occurring in the oxidation system. Steps resulting in the immediate production of acetylene seem to be molecular decomposition of molecules, or free radicals, or dehydrogenation, followed by combination or addition of oxygen. Peroxide formation may occur also. These reactions may be a general source of the hydrocarbon flame bands. 

In 1892, Lewes (13, 14, 66) first proposed that acetylene is an important intermediate in combustion processes. He suggested a theory of luminosity of flames based upon its formation and subsequent incandescent decomposition. This did not prove successful as a general explanation of luminosity, but in recent years a modified version of this concept has been advocated by Porter (58, 59) and others. They suggest that acetylene is the starting point for the major reactions of carbon formation in flames and combustion. 

Wright, F. J., "Carbon Formation Under Well Stirred Conditions. Part II." Combustion Flame 15, 217, 1970. 

There are a great many published reports describing studies of carbon black formation in flames. Many of these deal with gaseous fuels in either pre-mixed flames or diffusion flames. The principal objectives are to develop a better knowledge of combustion through an understanding of the kinetics and mechanism of carbon formation,... 

The measurements were made in the primary zone of carbon formation, i.e., above the flame front, in a region where Tp and Xo (or X) are practically constant and where there is no agglomeration. This latter point can be verified by Equation 1 which ceases to be valid when particle size is larger than about 500 A (7). The initial mixtures were chosen so that Xo could be varied in suflBcient proportions. The mixture compositions used are listed in Table I. 

Kerosene can vary widely in its burning quality as measured by carbon deposition, smoke formation, and flame radiation. This is a function of hydrocarbon composition—paraffins have excellent burning properties, in contrast to those of the aromatics (particularly the polynuclear aromatic hydrocarbons). As a control measure the smoke point test (ASTM D-1322, IP 57) gives the maximum smokeless flame height in millimeters at which the fuel will burn in a wick-fed lamp under prescribed conditions. The combustion performance of wide-cut fuels correlates well with smoke point when a fuel volatility factor is included, because carbon formation tends to increase with boiling point. A minimum smoke volatility index (SVI) value is specified and is defined as ... 

U burns m air witii a blue flame and formation of carbon dioxide it forms explosive mixtures with air and oxygen it is oxidized to carbon dioxide by cold chromic acid. It is a valuable reducing agent, and is used for the reduction of metallic oxides at a red heat. Anunoniacal solutions of the cuprous salts absorb it readily. Being nou-eaturated, it unites readily vrith O to form CO, and with Cl to fbrni the tafter a nnlnrleRa, mif-... 

Calcium hydroxide is an interesting material as, although appearing to have suitable properties, both Nishimoto and co-workers [24] and Ashley and Rothon [8] have found it to exhibit relatively poor flame retardant properties in the oxygen index and other tests. Ashley and Rothon showed that, in some polymers at least, the final product from the oxygen index test was calcium carbonate rather than the oxide. As carbonate formation is exothermic, they associated this with the poor performance. More recently, Miyata has claimed that doping with certain metals significantly improves the performance of calcium hydroxide [25], but this has not been verified. 

In order to maintain high energy efficiency and ensure a long service life of the materials of construction in the combustion chamber, turbine and jet nozzle, a clean burning flame must be obtained that minimizes the heat exchange by radiation and limits the formation of carbon deposits. These qualities are determined by two procedures that determine respectively the smoke point and the luminometer index. 

Carbon monoxide test. Warm together carefully 0 5 ml. of formic acid (or 0 5 g. of a formate) and i ml. of cone. 112804. Identify the carbon monoxide by igniting the gas evolved and observing the pale blue flame travel down the test-tube. Note that dilute solutions of formic acid will not give this test. HCOOH — HjO — CO. 

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). 

Soot. Emitted smoke from clean (ash-free) fuels consists of unoxidized and aggregated particles of soot, sometimes referred to as carbon though it is actually a hydrocarbon. Typically, the particles are of submicrometer size and are initially formed by pyrolysis or partial oxidation of hydrocarbons in very rich but hot regions of hydrocarbon flames conditions that cause smoke will usually also tend to produce unbumed hydrocarbons with thek potential contribution to smog formation. Both maybe objectionable, though for different reasons, at concentrations equivalent to only 0.01—0.1% of the initial fuel. Although thek effect on combustion efficiency would be negligible at these levels, it is nevertheless important to reduce such emissions. 

Pollutant Formation and Control in Flames Key combustion-generated air pollutants include nitrogen oxides (NOJ, sulfur oxides (principally SO9), particulate matter, carbon monoxide, and unburned hydrocarbons. 

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