Analysis of nitro- and oxy-PAH emissions from a pilot scale silicon process with flue gas recirculation†

Silicon alloys are produced by carbothermic reduction of quartz in a submerged arc furnace. This high-temperature pyrolytic process is a source of polycyclic aromatic hydrocarbons (PAHs), which are a group of aromatic organic molecules with known mutagenic and carcinogenic properties. In this study, the emission of oxy- and nitro-PAHs from a pilot-scale Si furnace, with varying process conditions such as oxygen level, flue gas recirculation (FGR), and off-gas flow, was investigated. Analysis shows the presence of both oxy- and nitro-PAH species in all experiments, believed to be formed from radical-induced substitution reactions initiated by SiO combustion and NO_x formation. During Si production without FGR, the levels of oxy- and nitro-PAHs range between 1.1 and 4.4 μg Nm⁻³, independent of the flue gas flow rate. With increasing FGR (0–82.5%) and decreasing oxygen level (20.7–13.3%), the concentrations of both oxy- and nitro-PAHs increase to 36.6 and 65.9 μg Nm⁻³, respectively. When the levels of substituted PAHs increase, species such as 4-nitropyrene and 1,2-benzanthraquinone are in abundance compared to their parent PAHs. Experiments at lower flue gas flow (500 Nm³ h⁻¹versus 1000 Nm³ h⁻¹) generally produce less substituted PAHs, as well as SiO₂ particulate matter and NO_x, where the latter two parameters have a 99% correlation in this study.