PAH and Soot Formation

Generation of Pollutants from Combustion 6.3.1 PAH and Soot Formation... 

PAHs and soot formation it cannot be excluded that a fraction of them, once formed, are converted into PAHs and soot during the arcing operation. 

Aromatic hydrocarbons are known to be important in soot formation in flames. The aromatic structure may abet molecular growth leading to PAH and soot formation through its ability to stabilize radicals formed from addition of aromatic radicals to unsaturated aliphatics such as acetylenic species (jL>2.). Accordingly, both aromatics and unsaturated aliphatics would be important for growth processes. Both types of species are prevalent in the flame zone where growth occurs. Aromatic structures with unsaturated side chains also are observed there (1 >3). 

Major unknowns in the mechanism by which a hydrocarbon fuel bums concern the pyrosynthesis reactions that lead to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot and the oxidation chemistry of atoms other than carbon and hydrogen (heteroatoms) in the fuel, particularly nitrogen, sulfur, and halogens. 

The formation of PAHs and soot involves a molecular precursor stage in which hydrocarbon radicals, especially with 2-5 carbon atoms, play a role to build up... 

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 ... 

High-temperature pyrolysis reactions of hydrocarbons are responsible for the production of PAH and solid carbon black particles, soot. This phenomenon is common in diffusion flames where, at high temperatures and without oxygen, hydrocarbon fuel aggregates follow pyrolysis and condensation paths with the formation of heavy aromatic structures. Many PAH s identified in aerosols have been found to be mutagenic and are certainly important soot precursors. This formation of carbonaceous particles has recently become one of the main topics in chemical reaction engineering, especially in the field of pyrolysis and combustion of hydrocarbon fuels. This interest rises from environmental concerns about PAH and soot particle emissions because of their dangerous impact on the human health (Oberdorster et al., 2004). 

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... 

In the mid-1980s, the discovery of significant amounts of 2-nitropyrene in ambient particulate matter collected in a rural region of Denmark by Nielsen and co-workers (1984) and 2-nitrofluoranthene in southern California by Pitts and colleagues (1985b) provided unique initial evidence for the possible formation of nitroarenes by reactions of precursor PAHs in ambient air. Thus, these compounds are not electrophilic nitration products of their parent PAHs and are generally not observed in combustion sources such as diesel soot see, for example, Table 10.34 and the report of Ciccioli and co-workers on the detection of emissions of 2-nitrofluoranthene and 2-nitropyrene solely from a very minor Italian industrial source (see Ciccioli et al., 1993, 1995, 1996, and references therein). 

Density functional theoretical calculations were applied to the formation of internal primary ozonides from three PAHs (pyrene, coronene, and circum-pyrene O Hm) to simulate the atmospheric interaction between ozone and soot. No 1,2,4-trioxolane intermediate was considered in the conversion of the 1,2,3-trioxolane into aromatic epoxides via ring-opened trioxyl diradicals <2005PCA10929>. 

One of the drawbacks of the synthesis of the polyynes with the submerged carbon arc is the simultaneous formation of a mixture of PAHs (polycyclic aromatic hydrocarbons) and soot (see Chapter 8). Apparently the PAHs... 

Aromatic compounds have not only been of academic interest ever since organic chemistry became a scientific discipline in the first half of the nineteenth century but they are also important products in numerous hydrocarbon technologies, e.g. the catalytic hydrocracking of petroleum to produce gasoline, pyrolytic processes used in the formation of lower olefins and soot or the carbonization of coal in coke production [1]. The structures of benzene and polycyclic aromatic hydrocarbons (PAHs) can be found in many industrial products such as polymers [2], specialized dyes and luminescence materials [3], liquid crystals and other mesogenic materials [4]. Furthermore, the intrinsic (electronic) properties of aromatic compounds promoted their use in the design of organic conductors [5], solar cells [6],photo- and electroluminescent devices [3,7], optically active polymers [8], non-linear optical (NLO) materials [9], and in many other fields of research. 

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... 

Soot forms in a flame as the result of a chain of events starting with the oxidation and/or pyrolysis of the fuel into small molecules. Acetylene, C2H2, and polycyclic aromatic hydrocarbons (PAHs) are considered the main molecular intermediates for soot formation and growth (McKinnon and Howard 1990). The growth of soot particles involves first the formation of soot nuclei and then their rapid growth due to surface reactions (Harris and Weiner 1983a,b). 

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