1. Classification and Properties

Polycyclic Aromatic Hydrocarbons ([GLOSS]PAH[/GLOSS]s) are a sizable group of compounds consisting only of carbon and hydrogen (therefore being [GLOSS]organic[/GLOSS]), with two or more condensed [GLOSS]aromatic[/GLOSS] rings. They play an important role in the assessment and control of air quality as they include a large number of chemicals that can be considered hazardous for the environment and for human health. For this reason, they are strictly regulated by law in most industrialised countries.

PAHs are generally considered to be Semi-Volatile Organic Compounds ([GLOSS]SVOC[/GLOSS]s); their volatility is strongly related to the number of aromatic rings as well as the molecular structure. Naphthalene (two benzenoid rings), the simplest PAH, has the lowest boiling point (218^oC) and is relatively volatile, while PAHs with more than 5 rings are considered scarcely volatile, as boiling points generally increase with the number of rings.

PAHs have low vapour pressures and are found at ambient temperature in air both as vapours and associated with particles. They are relatively insoluble in water, but dissolve easily in fats and oils. In combustion systems, they are typically present as gases but can be emitted from the stack both as vapour and particulate matter.

Some of the PAHs more frequently tested for in emissions are Benzo[a]pyrene ([GLOSS]B[a]P[/GLOSS]), Acenaphthene, Dibenzo(a,h)anthracenes, Anthracene, Fluoranthene, Fluorene, Naphthalene, Phenanthrene, Chrysene and Pyrene. The chemical structures of some of them are illustrated in Figures 1-4.

2. Importance

In relation to [GLOSS]combustion[/GLOSS], PAHs are important as they are precursors of soot and can also play a role in the formation of other organic micropollutants (i.e. dioxins, [GLOSS]furans[/GLOSS] and other aromatic species of environmental concern). Their main concern, however, is as toxic combustion by-products, as some of them can exhibit [GLOSS]carcinogenic[/GLOSS], [GLOSS]teratogenic[/GLOSS] and [GLOSS]mutagenic[/GLOSS] properties. Their complex nature and the high number of compounds belonging to this class makes an exact definition of the relative importance of the role of each one of them very difficult. It is for this reason that in certain countries their emissions are limited by law as a whole class, without specifying single chemical species (i.e. U.S.A.-EPA Clean Air Act 1990).

3. Sources of PAHs

PAHs can be formed both from natural and [GLOSS]anthropogenic[/GLOSS] sources.

Natural sources may include thermal geological processes, natural fires and biosyntheses.

The main anthropogenic sources of PAHs are considered to be stationary and mobile combustion processes, such as fossil, bio- and waste-derived fuel combustion, road transport emissions, industrial and chemical processes for heat and power generation, and waste incineration. PAHs at trace level can be produced, however, from any kind of combustion process, such as cigarette smoking, grilling of meat and burning of any other carbonaceous material.

The detailed formation pathway of PAHs during combustion is still not completely understood. However, it is known that PAH can be formed through complex homogeneous phase reactions of radicals and stable organic compounds, including single ring aromatic structures ([GLOSS]MAC[/GLOSS]); these processes occur mainly in the [GLOSS]pyrolytic microzones[/GLOSS] of the flame; the formed PAHs can be destroyed in further oxidative steps of combustion, but some of them can survive, even though at trace level, being emitted at the stack.

4. Typical Emissions

Generally, emissions are greater from [GLOSS]pyrolysis[/GLOSS] processes, and least from pulverised [GLOSS]coal[/GLOSS] combustion units. This subject shall be dealt with in greater detail in further Combustion Files.

Typical emissions of PAHs from various furnace types are listed in Table 1.

Table 1. Typical PAH and B[a]P emissions from various furnace types (Zevenhoven and Kilpenen, 2001; data from Huotari and Vesterninen, 1995)

5. Potential Control

Potential control of PAHs is both through reduction of their formation through combustion modification, and through gas cleanup. Formation reduction is through process optimisation and increasing combustion efficiency as PAH can be definitely considered as a special class of Products of Incomplete Combustion ([GLOSS]PIC[/GLOSS]s) . Plant design, scale up, temperature, residence time and turbulence are therefore important parameters to be considered to reduce their primary formation.

Gas cleanup options are through thermal or catalytic oxidation (though catalyst deactivation may be a problem with hydrocarbons), or the use of [GLOSS]activated carbon[/GLOSS] beds.

6. Health and Environmental Effects

Health Effects

It is not clear that PAHs cause short-term health effects. Long-term health effects of exposure to PAHs may include cataracts, kidney and liver damage, and jaundice. Repeated skin contact to naphthalene can result in redness and inflammation of the skin. Breathing or swallowing large amounts of naphthalene can cause the breakdown of red blood cells.

Long-term exposure to low levels of some PAHs have caused cancer in laboratory animals. Benzo(a)pyrene is known to cause cancer in experiment animals. Studies of workers exposed to mixtures of PAHs and other compounds have noted an increased risk of skin, lung, bladder, and gastrointestinal cancers. A number of individual PAHs are classified by the International Agency of Research on Cancer as carcinogenic to animals and probably carcinogenic to humans.

Exposure is through the anthropogenic sources mentioned above – exposure is greatest for workers in industrial and chemical processes. General exposure is through cigarette smoke, car exhaust gases and food.

It is difficult to quantify exposure, due to the large number of PAHs. Benzo[a]pyrene -B[a]P has been chosen as a marker for the total mixture of PAHs in the United Kingdom.

The level recommended by the Expert Panel on Air Quality Standards (United Kingdom) for B[a]P as a marker for total PAHs is 0.25 ng/m³ measured as an annual average.

Environmental Effects

Environmental effects are mainly related both to the carcinogenic properties of some PAHs and their reactivity in the environment. They can be transformed in the atmosphere  through  photo-oxidative reactions in different species, some hazardous, some innocuous. Some processes occurring in the atmosphere, in soil or aquatic environments are also able to destroy them, avoiding their accumulation when the emission rates are low enough. Contamination of the environment with PAHs increases risk exposure of aquatic and land-based animal species and thereby threatens biodiversity. Consumption of fish or livestock products with increased PAH levels puts humans at higher risk of health effects.

Sources

[1] Zevenhoven, R. and Kilpinen, P., Control of Pollutants in Flue Gases and Fuel Gases, Helsinki, 2001

[2] Department of the Environment, Transport and the Regions (UK). Expert Panel on Air Quality Standards 9th Report. Polycyclic Aromatic Hydrocarbons. SO 1999. ISBN 0 11 7535036

[3] Agency for Toxic Substances and Disease Registry (US Government). ToxFAQ on Polycyclic Aromatic Hydrocarbons, September 1996. Accessed October 22, 2002, at:

http://www.atsdr.cdc.gov/tfacts69.html

[4] Huotari, J. and Vesterninen, R. “Muut polton päästöt, Chapter 11.” IFRF Finland, 1995.

[5] Richter, H. and Howard, J.B. Formation of polycyclic aromatic hydrocarbons and their growth to soot – a review of chemical reaction pathways, Progress in Energy and Combustion Science, Volume 26, 2000.