Formation of Polycyclic Aromatic Hydrocarbons (PAH) in Hydrocarbon Combustion: Comparative New Results from Premixed Flames

Here is a recent publication from our group on this subject.

One facet of our current research at the CRE laboratory is the production of polycyclic aromatic hydrocarbons (PAH) in hydrocarbon combustion. A series of premixed flame experiments have been performed using direct GC/MS gas analysis to determine the micro-structures of methane, ethane, propane, butane, and ethylene flames. In each case over 40 major, minor and trace species were quantified including a large number of aromatics and PAH. However, due to space limitations, only the comparative results of selected species are shown for the simple, alkane, premixed flames. Flame parameters, such as equivalence ratio, carbon density, H/C ratio, were chosen in this study and are summarized in the Table below.

In summary, the direct sampling and GC/MS analysis of a fuel-rich, laminar, premixed flame of methane indicates the production of significantly higher in-flame peak concentrations of benzene and polycyclic aromatic hydrocarbons (PAH) than in a flame of ethane under similar combustion conditions, in spite of the higher H/C ratio and lower carbon density of the former.

Comparative Results of Benzene Formation in Premixed Hydrocarbon Flames

These findings are surprising, as methane is generally believed to be the cleanest burning hydrocarbon fuel and natural gas is considered to be the fuel of the future. These results also suggest the importance of species containing an odd number of carbon atoms in PAH formation. Although aromatic and polyaromatic intermediates constitute trace by-products of combustion, their formation is of practical concern due to their potential adverse health effects.

Results of Naphthalene Production in Hydrocarbon Premixed Flames

The major chemical features of methane combustion have been studied extensively in the past both in premixed and diffusion flames. In a limited number of studies, the concentrations of some PAHs were also measured, but not by a direct analysis technique. Similarly, the chemical structures of ethane and propane flames were also investigated, again with emphasis of major products formed. Surprisingly, however, PAH levels in methane flames have not been compared to other hydrocarbon fuels under similar combustion conditions to assess the relative emission rates of PAH pollutants. Comparative results on PAH formation in hydrocarbon flames are provided in the list of images below:

Phenanthrene Formation in Premixed Hydrocarbon Flames
Pyrene Formation in Premixed Hydrocarbon Flames
Temperature Profiles in Premixed Hydrocarbon Flames

Methane, the major component in natural gas, is generally believed to be the cleanest burning hydrocarbon as it produces less CO₂ and more H₂O than other fossil fuels because of its high H/C ratio. Another important aspect of methane combustion which contributed to its pristine image is the fact that its premixed flames with air do not soot, instead they are blown-out first because of flammability limit considerations. However, in many other applications, such as in industrial power generators, heaters and boilers, hydrocarbons are burned in diffusion type flames, where fuel concentrations span a wide range from fuel-rich to fuel-lean conditions. In fuel-rich regions the production of undesirable by-products including PAH can occur. These pollutants can then be transported out of the main combustion core and emitted to the atmosphere. As stated previously, because many of the PAHs formed are potentially toxic, carcinogenic or mutagenic, it is of considerable practical interest to compare the nature and levels of these by-products in methane combustion in relation to those observed in the burning of other fossil fuels. In addition, there is growing interest to use natural gas as a supplement or a replacement for other fuels to reduce the emission of toxic by-products.

PAH levels in flames are conventionally determined by pre-concentrating them on adsorbates such as Tenax and XAD resins, followed by solvent extractions, re-concentration and GC/MS or HPLC analysis, which collectively can take anywhere from 24 to 48 hours to complete. In addition, the numerous sample transfer steps involved can result in PAH loss, thereby decreasing the reliability of these measurements. Recognizing these limitations, we developed a new accurate method that avoids most of the sample transfer steps associated with conventional methods. This new, accurate technique is visualized in the image below.

Experimental system for the analysis of flame gases from a premixed hydrocarbon flame

In this approach, hot gases bearing PAHs are withdrawn from within the flame using a heated probe and transported to and directly analyzed by GC/MS. This direct analysis approach permits the accurate determination of the identities and absolute concentrations of PAHs rapidly, in 30 minutes or less, and represents an improvement of productivity by nearly a factor 20.

Here is a recent publication from our group on this subject.

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