Combustion polycyclic aromatic hydrocarbon formation

N.M. Marinov, M J. Castaldi, C.F. Melius, and W. Tsang. Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Premixed Propane Flame. Combust. Sci. Techn., 128 295-342,1996. 

Marinov, N.M. Pitz, W.J. Westbrook, C.K. et al. Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame. Combust. Flame 1998, 114, 192-213. 

FIGURE 7.9 Mechanism of formation of polycyclic aromatic hydrocarbons (PAHs) during combustion. 

Richter, H. and Howard, J.B., Formation of polycyclic aromatic hydrocarbons and their growth to soot—a review of chemical reaction pathways, Prog. Energy Combust. Sci., 26,565,2000. 

The recommended method of trichloroethylene disposal is incineration after mixing with a combustible fuel (Sittig 1985). Care should be taken to carry out combustion to completion in order to prevent the formation of phosgene (Sjoberg 1952). Other toxic byproducts of incomplete combustion include polycyclic aromatic hydrocarbons and perchloroaromatics (Blankenship et al. 1994 Mulholland et al. 1992). An acid scrubber also must be used to remove the haloacids produced. 

One of the more significant classes of compounds resulting from and emitted by combustion sources include polycyclic aromatic hydrocarbons (PAHs) these species serve as nuclei for the formation of soot particles. Past studies have concluded that 85% of... 

PCDD/F and other chlorinated hydrocarbons observed as micropollutants in incineration plants are products of incomplete combustion like other products such as carbon monoxide, polycyclic aromatic hydrocarbons (PAH), and soot. The thermodynamically stable oxidation products of any organic material formed by more than 99% are carbon dioxide, water, and HCl. Traces of PCDD/F are formed in the combustion of any organic material in the presence of small amounts of inorganic and organic chlorine present in the fuel municipal waste contains about 0.8% of chlorine. PCDD/F formation has been called the inherent property of fire. Many investigations have shown that PCDD/Fs are not formed in the hot zones of flames of incinerators at about 1000°C, but in the postcombustion zone in a temperature range between 300 and 400°C. Fly ash particles play an important role in that they act as catalysts for the heterogeneous formation of PCDD/Fs on the surface of this matrix. Two different theories have been deduced from laboratory experiments for the formation pathways of PCCD/F ... 

For soybean-based biodiesel at this concentration, the estimated emission impacts for percent change in emissions of NO,, particular matter (PM), HC, and CO were +20%, -10.1%, -21.1%, and -11.0%, respectively (EPA, 2002). The use of blends of biodiesel and diesel oil are preferred in engines in order to avoid some problems related to the decrease of power and torque, and to the increase of NO, emissions (a contributing factor in the localized formation of smog and ozone) that occurs with an increase in the content of pure biodiesel in a blend. Emissions of all pollutants except NO appear to decrease when biodiesel is used. The use of biodiesel in a conventional diesel engine dramatically reduces the emissions of unbumed hydrocarbons, carbon dioxide, carbon monoxide, sulfates, polycyclic aromatic hydrocarbons, nitrated polycyclic aromatic hydrocarbons, ozone-forming hydrocarbons, and particulate matter. The net contribution of carbon dioxide from biomass combustion is small. 

Before we examine the oxidation pathways available to aromatic systems, it is first instructive to review the most notorious role of these compounds in combustion chemistry their propensity to lead to soot formation. Soot is a byproduct of fuel-rich combustion, and soot particles can affect respiration and general health in humans." Soot production is a result of polycyclic aromatic hydrocarbon (PAH) formation in flames as reactive hydrocarbon radical intermediates combine to grow... 

Thermochemical data are available (Ref 2) on the heats of combustion and formation for all five isomers, on the heats of nitration from various Dinitrotoluenes for the 23,4-, 2,4,5-, and 2,3,6-isomers, and on the heats of crystn for the 2,3,4- and 2,4,5-isomers. Data are also available (Ref 1) on the shock sensitivities of all of the isomers except 2,3,6-, and on the rates of decompn at 140° of the 23,4-, 2,4,5-, and 23,5-isomers. The detonation pressure and the temp coefficient of decompn between 140 and 180° have been measured for the 2,4,5-isomer 2,3,4- and 2,4,5-TNT form addition compds ( 7r-complexes ) at 1 1 molar ratio with several polycyclic aromatic hydrocarbons (naphthalene, acenaphthene, fluorene, phenanthrene and anthracene) (Ref 2). 2,4,5-TNT forms complexes with 4-aminozaobenzene, 4-aminoacetophenone, bis (2 hydroxy ethyl) amine, and tris (2-hydroxy-ethyl) amine (Ref 1). The first two have a 1 1 molar ratio, the third 1 2, and the fourth 2 1. Upon heating, the two 4-amino compds react with replacement of the 5-nitro group, as discussed below... 

The past decade has led to the detection of new carbon allotropes such as fullerenes26 and carbon nanotubes,27 28 in which the presence of five-mem-bered rings allows planar polycyclic aromatic hydrocarbons to fold into bent structures. One notes at the same time that these structures are not objects of controlled chemical synthesis but result from unse-lective physical processes such as laser ablation or discharge in a light arc.29 It should be noted, on the other hand, that, e.g., pyrolytic graphitization processes, incomplete combustion of hydrocarbon precursors yielding carbon black, and carbon fibers30 are all related to mechanisms of benzene formation and fusion to polycyclic aromatic hydrocarbons. 

Mere destruction of the original hazardous material does not adequately measure the performance of an incinerator. Products of incomplete combustion can be as toxic as, or even more toxic than, the materials from which they evolve. Indeed, highly mutagenic polycyclic aromatic hydrocarbons are generated in the fuel rich regions of most hydrocarbon flames [48]. Dioxin formation in the combustion of chlorinated hydrocarbons has also been reported [49],... 

The chemical dynamics, reactivity, and stability of carbon-centered radicals play an important role in understanding the formation of polycyclic aromatic hydrocarbons (PAHs), their hydrogen-dehcient precursor molecules, and carbonaceous nanostructures from the bottom up in extreme environments. These range from high-temperature combustion flames (up to a few 1000 K) and chemical vapor deposition of diamonds to more exotic, extraterrestrial settings such as low-temperature (30-200 K), hydrocarbon-rich atmospheres of planets and their moons such as Jupiter, Saturn, Uranus, Neptune, Pluto, and Titan, as well as cold molecular clouds holding temperatures as low as 10... 

At low temperatures, in situations where there is relatively little 02, pyrolysis reactions (i.e. reactions where decomposition takes place as a result of heat) may cause a rearrangement of atoms that can lead to the formation of polycyclic aromatic hydrocarbons (see Section 2.7) during combustion. The most notorious of these is benzo[ ]pyrene (B[ ]P see Fig. 2.4), a cancer-inducing compound. 

Huang, J.W. Senkan, S.M. Polycyclic aromatic hydrocarbon and soot formation in premixed flames of CH3CI/CH4 and CH4. 26th Symposium (Int l) on Combustion. The Combustion Institute Pittsburgh, 1996 2335-2341. 

Is it fascinating to know that the polyynes are considered the key precursors in the formation of polycyclic aromatic hydrocarbons and soot in combustion flames and that a similar mechanism of carbon dust formation can be applied (without flames) to explain the formation and the abundance of elemental carbon in our galaxy. Carbon dust represents about 3% of the... 

Soot emitted from Diesel engines is hazardous for human health since it is made of inhalable particles [1] and contains gases and liquids adsorbed on its smrface, some of which (Polycyclic Aromatic Hydrocarbons) are suspected to be cancerogenic [2]. Virtually, soot-free Diesel exhaust may be obtained combining reduction of soot formation in the combustion chamber with exhaust gas treatment [3]. This latter is generally performed by a ceramic wall-flow filter that collects the carbonaceous particles while the filter regeneration is achieved by post-combustion of collected soot [3, 4]. 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

A number of analytical methods [66, 67] involving pyrolysis of polymers have been reported in the literature. Michal and co-workers [68] developed a method using direct gas chromatography (GC)-mass spectrometry (MS) for their study of the combustion of polyethylene (PE) and PP. Morikawa [69] used GC to determine polycyclic aromatic hydrocarbons in the combustion of polymers. Liao and Browner [70] also described a method for the determination of polycyclic aromatic hydrocarbons. Many other workers have studied soot and smoke formation and their mechanisms in the combustion of polymers. Generally in these studies, relatively simple and specific methods were used, which were appropriate for the intended tasks. However, these methods are not suitable for complete analysis of the very complex smoke particulates resulting from combustion of many polymers. Most methods have been developed either for volatile compounds of low molecular weight or for polycyclic aromatic hydrocarbons. Joseph and Browner [71] developed a method that can be used to... 

The sub-stoichiometric combustion involves the risk of soot formation as a result of pyrolysis reactions with acetylene and polycyclic aromatic hydrocarbons as soot precursors [107] [465]. The soot formation will start below a certain steam-to-carbon ratio depending on pressure and other operating parameters [111]. However, the data in Table 1.8 shows results from a soot-free pilot test (100 Nm NG/h) at a low steam-to-carbon ratio of 0.21 [111]. 

Large amounts of smoke from oil slick burning can result in oil rain. The formation and possible sinking of extremely viscous and dense residues can damage the sea bed and its inhabitants. The viscous residue may also be transported to shorelines and beaches by ocean tides or currents. Airborne irritants and possibility of secondary fire are sources of concern when combustion has to be carried out close to residential areas. Carbon monoxide, sulfur dioxide, and polycyclic aromatic hydrocarbons (PAH) are common toxic compounds emitted while burning oil on water. 

Phenyl radicals have an important role to play in the combustion of fossil fuels and in the formation of polycyclic aromatic hydrocarbons. The absolute rate constant for the reaction of CeHs with 2-methylpropane, 2,3-dimethylbutane and 2,3,4-trimethylpentane has been measured using cavity ring-down spectromehy between 290 and 500 K. The reactions were found to be dominated by the extraction of H atoms from the tertiary C—H bonds. ... 

Nitro-substituted polycyclic aromatic hydrocarbons (nitro-PAHs) are formed during the combustion of fossil fuels at high temperatures with a vast supply of combustion air. In this reaction, conversion of nitrite (NO2) to nitric acid is an important intermediate step. Another source of nitro-PAHs is the photochemical radical-mediated conversions of parent PAHs to nitro-derivatives. Combustion at high temperatures with a vast supply of combustion air may lead to the formation of 1-nitropyrene (1-NP), whereas photochemical conversion of pyrene gives rise to 2- and 4-nitropyrene [24,25]. 

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