Coal tar pitch, a well-known source of polycyclic aromatic
hydrocarbons
(PAHs), is used as a binder of petroleum coke in prebaked anodes used
for electrolysis of aluminum. Anodes are baked up to 1100 °C
over a 20-day period, where flue gas containing PAHs and volatile
organic compounds (VOCs) are treated using techniques such as regenerative
thermal oxidation, quenching, and washing. Conditions during baking
facilitate incomplete combustion of PAHs, and due to the various structures
and properties of PAHs, the effect of temperature up to 750 °C
and various atmospheres during pyrolysis and combustion were tested.
PAH emissions from green anode paste (GAP) dominate in the temperature
interval of 251–500 °C, where PAH species of 4–6
rings make up the majority of the emission profile. During pyrolysis
in argon atmosphere, a total of 1645 μg EPA-16 PAHs are emitted
per gram of GAP. Adding 5 and 10% CO2to the inert atmosphere
does not seem to affect the PAH emission level significantly, at 1547
and 1666 μg/g, respectively. When adding oxygen, concentrations
decreased to 569 μg/g and 417 μg/g for 5% and 10% O2, respectively, corresponding to a 65% and 75% decrease in
emission.