PAH emissions from the metallurgical industry

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic molecules consisting of two or more fused aromatic rings. They can be created by incomplete combustion of organic materials and accumulate in nature. As a group, they are considered to be hazardous to human health as some PAH molecules are proven to be carcinogenic. Reduced PAH exposure is therefore recommended by the Norwegian government, among others. Through the use of carbon materials, the metallurgical industry emits PAH from different sources. Emissions in Norway have been reduced over the last decades, with the airborne persistent organic pollutant PAH emissions being 5.14 tonnes in 2020. Due to PAHs ability to accumulate in the environment further emission reduction is needed.

The main objective of this work has therefore been to contribute to the current knowledge on the influence of process conditions on PAH behaviour in metallurgical systems through laboratory and pilot-scale experiments. Special attention was given to investigating an extended range of PAH species, outside the standard list of PAH-16, and processes related to aluminium electrolysis and the production of metallurgical-grade silicon. During laboratory experiments, varying atmospheres and temperatures were used to investigate the variations of PAH emissions from green anode paste, a mixture used to produce pre-baked anodes for aluminium electrolysis. In the pilot-scale campaign for silicon production with flue gas recirculation (FGR), the influence of FGR on PAH formation and destruction was in focus. The last activity looked into the existence and formation of substituted PAHs from silicon and aluminium-related processes, where oxygenated and nitrated PAHs were of interest for the silicon process and sulfonated PAHs for the aluminium industry.

It was concluded from the laboratory study with green anode paste that the O2 atmosphere showed the most influence on the emission levels, compared to argon and CO2 atmospheres and emissions being at the highest level at temperatures close to 500 °C (with 750 °C being the maximum temperature tested). Four and five-ringed PAH dominated the emissions profile and even though the total emission level was reduced by the oxygen atmosphere, the level of four-ring PAHs was stable for all atmospheres as a result of varying molecular reactivity.

Through the silicon pilot-scale study, the PAH emissions were influenced by the changing process conditions caused by FGR. Both the level of FGR and oxygen correlated with the PAH emissions, causing the emission level to rise with increased FGR and decreased oxygen concentration. The measurements showed increased emissions of PAHs with lower molecular weight (bicyclic species), species of high thermal stability and the presence of substituted PAHs, where both oxy- and nitro-PAH species were found in greater amounts than their parent PAHs at elevated FGR levels. In addition, through combined characterisation and analytical methods related to sulfonated PAHs, the work concluded by finding the presence of polar polycyclic aromatic compounds, containing sulfate and sulfonated species, which previously have not been reported for PAH-related industry emissions.