Particulate-bound PAHs, nitro and oxy-PAHs from small scale boilers fueled with non-woody biomass

Polycyclic Aromatic Hydrocarbons (PAHs) and their derivatives (oxy-PAHs and nitro-PAHs) are released during the biomass combustion process, both in the exhaust gas and bound to the particulate matter (PM). The composition of PAHs, including the number of carbon atoms and their molecular weight, is directly related to their toxicity. The aim of this work was to determine the PM-bound emission of the 16 EPA priority PAHs and some of their nitrated and oxygenated derivatives from the combustion of exhausted olive cake (EOC) and almond shell (AS) in a domestic boiler at two loads, nominal (55 kW) and partial (24 kW). In addition, pine pellet (P) was investigated as a reference fuel. Total PAHs emissions ranged from 1.7 to 8.4 μg/kg and from 5.7 to 357.8 μg/kg at nominal and partial load, respectively. While AS was the most PAHs-forming fuel at nominal load, EOC caused the highest emission at low load. The influence of the boiler load was quite relevant regarding the molecular weight of PAHs bound to PM. At nominal load, EOC and AS were dominated by low molecular weight compounds (LMW) and high molecular weight compounds (HHW), respectively, while at partial load medium molecular weight (MMW) PAHs gained importance for AS (59 %) and EOC PM were clearly dominated by high molecular weight PAHs (99 %). The higher unburned matter at partial load led to higher oxy and nitro-PAHs concentrations for the three biomass fuels, being in all cases 3 to 4 orders of magnitude lower than their derivative PAHs. EOC at partial load presented the highest value of KE sum and therefore its use in a modular boiler is less recommended from the health risk point of view. • Almond shells led to a PAHs emission factor of ∼8 μg/kg, four times higher than pine. • The PAHs emission factor for exhausted olive cake was similar to that of pine (∼2 μg/kg). • The PAHs distribution was highly dependent on the boiler load. • Oxy and nitro-PAHs were more significant at low load. • The carcinogenic equivalent was higher at partial load, especially for EOC, with a value around 43.