# Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers
## 2 Literature Review
### Origin, Generation and Fate of PCDD/Fs and PAHs in Combustion Processes
Fuel contaminants, such as creosote (a wood preservative), can be important sources of PAHs in combustion systems [61]. PAHs formed in biomass combustion appear to be mostly associated with gas emissions and soot deposits; while LPAHs (two to three rings) are preferentially present in gas phase emissions, HPAHs (more than three rings) are more associated with ashes [33; 221; 222]. [PERSON] and [PERSON] [61] detected PAHs contents as high as 77 mg/kg, of which 11 mg/kg were carcinogenic, in ashes from a biomass boiler. [PERSON] et al. [50] found 0.18-2.61 mg/kg of PAHs in bottom ashes, and 3.59-193 mg/kg in fly ashes from combustion of several biomass residues. LPAHs were preponderant, with NaP accounting for 50-90% in fly ashes, while in bottom ashes more of the three, four and five rings were detected. BaP and other carcinogenic PAHs were found below 0.51 mg/kg in both ashes. [PERSON] and [PERSON] [53] detected 6.16 mg/kg of PAHs in a biomass ash containing unburned matter (14.5%), and thus suggested the need to measure PAHs in ashes for soil application, when unburned matter exceeds 9%. However, high PAHs, including the carcinogenic ones, were found in porous fly ash particles from rice husk combustion, independently of their unburned content [223]. [PERSON] [51] detected high contents of PAHs, 91-300 mg/kg, in biomass fly ashes of winter seasons, containing 25-45% unburned carbon. NaP was prevalent (44-120 mg/kg), followed by Acy, Phe, Pyr and Flut, and the carcinogenic IP and BaP reached 7.7 and 3.9 mg/kg, respectively. [PERSON] et al. [49] found relatively low levels of PAHs in biomass ashes, although 0.4 mg/kg were present in bottom ashes containing 26% organic matter. Correlation of PAHs contents in ash with their unburned carbon content are commonly referred [26]. [PERSON] et al. [60] found exponential correlations of PAHs with unburned matter (loss in ignition-LOI) for wood fly ashes containing 3.8-148 mg/kg of PAHs. However, the levels of LOI were unusually high, reaching 55%. [PERSON] et al. [224] showed straight correlations between PAHs, PCNs and PCDD/Fs contents in waste incineration, suggesting formation by common reactions. [PERSON] and [PERSON] [225] showed that PCDDs and PCDFs may be generated from anthracene and 2-chloroanthracene, at 250-300 \({}^{\circ}\)C, in the presence of KCl and CuCl\({}_{2}\), over alumina. Moreover, [PERSON] et al. [226] revealed that some PAHs compounds produce PCDFs and PCNs, at 200-400 \({}^{\circ}\)C, in the presence of O\({}_{2}\). Also, it was demonstrated that PCDD/Fs with a high F/D ratio can be produced by heating various PAHs at 300 \({}^{\circ}\)C, with CuCl\({}_{2}\) as a catalyst; perylene being the most efficient [227]. [PERSON] et al. [228] suggested that the different patterns of PCDF isomers observed for stoker furnaces and fluidized bed systems could be attributed to their formation from PAHs with perylene structures. [PERSON] et al. [229] also found that some PAHs usually present in fly ash; fluorene, fluoranthene, phenanthrene and anthracene, originate PCDD/Fs at 250 \({}^{\circ}\)C, preferentially PCDFs. To effectively decrease yields of PCDD/Fs in biomass boilers, in addition to optimization of combustion conditions and use of adequate flue gas treatment, the composition of fuels is of prime interest. Besides controlling contents of metals and Cl (and other halogens), the pre-existence of PCDD/Fs in fuels must be avoided, as well as the absence of chemicals used as wood preservatives, pesticides and fertilizers that may serve as PCDD/Fs precursors. Hence, inclusion of treated waste wood and other wastes in biomass boilers should be avoided to prevent formation of very toxic unintentional POPs, such as PCDD/Ds, PCBs and carcinogenic PAHs.