What are Polycyclic Aromatic Hydrocarbons (PAHs)? A Combustion File downloaded from the IFRF Online Combustion Handbook ISSN 1607-9116 Combustion File No:

190

Version No:

1

Date:

10-12-02

Author(s):

Patrick Lavery

Source(s):

See CF

Sub-editor:

Neil Fricker

Referee(s):

Loretta Bonfanti

Status:

Published

Sponsor:

University of Glamorgan

1. Classification and Properties Polycyclic Aromatic Hydrocarbons (PAHs) are a sizable group of compounds consisting only of carbon and hydrogen (therefore being organic), with two or more condensed aromatic 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 (SVOCs); 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 (218oC) 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 (B[a]P), 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.

Figure 1. Pyrene

Figure 3. Fluoranthene

Figure 2. Chrysene

Figure 4. Acenaphthene

2. Importance In relation to combustion, 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, furans and other aromatic species of environmental concern). Their main concern, however, is as toxic combustion by-products, as some of them can exhibit carcinogenic, teratogenic and mutagenic 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 anthropogenic 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 (MAC); these processes occur mainly in the pyrolytic microzones 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 pyrolysis processes, and least from pulverised coal 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. Small wood stove

Small solid fuel furnace

Small residential furnace

Heating Furnace 1-5 MW

Heat and power units >5 MW

PAHs

100 - 1000

(µg/MJ)

1000 – 3000 (batch)

< 1000

2 – 10 (solid fuel)