# 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  
Several tragic historical events like the Agent Orange in Vietnam and Seveso accident [106; 107; 108], and more recently animal feed contamination with biodiesel sub-products [109], brought the PCDD/Fs issue to worldwide research focus, regulation, and continuous surveillance. The formation of PCDD/Fs in the combustion process was firstly reported in 1977 for waste incineration systems [110]. Since then, PCDD/Fs have been broadly investigated, despite the complexity and high costs associated with their characterization. Pre-existing PCDD/Fs in materials used as fuels, such as PCDD/F contaminated PCP treated wood, may be a source of emissions, for complete destruction is achieved only in high efficient combustion systems, with long residence times, typically above 1000 \({}^{\circ}\)C, and dedicated flue gas treatment. In combustion processes, however, PCDDs and PCDFs can also be formed through two pathways: primary formation by the precursor's mechanism, and secondary formation by de novo synthesis. The two mechanisms operate simultaneously and independently, in different zones of the combustion systems, and depend on various parameters like fuel composition (mainly Cl, Cu and S) and presence of organic contaminants, furnace conditions and flue gas treatment applied [111; 112; 113; 114; 115; 116; 117]. In the hottest furnace zones (>650 \({}^{\circ}\)C) and low O\({}_{2}\), PCDD/Fs originate from precursors like lignin of biomass, waste wood preservative additives containing pentachlorophenol (PCP), PCBs, and other compounds, such as chlorobenzes, chlorophenols and PAHs, which may be also formed during poor combustion [115; 118]. In the post combustion zones, like ESP and baghouse filters, de novo synthesis involves catalyzed chlorination of carbonaceous structures present in fly ashes, between 250-500 \({}^{\circ}\)C or higher, with a peak at 300 \({}^{\circ}\)C, Cu being the more relevant catalyzer [111; 119; 120; 121; 122; 123]. Studies on combustion of salty laden wood revealed that 99% of total PCDD/Fs were associated to cyclone and ESP fly ashes, and their main formation was suggested to be de novo synthesis over fly ashes [124]. Tests performed between 260-430 \({}^{\circ}\)C, revealed that PCDD/Fs formation decreases for lower temperatures and shorter residence times [125; 126]. Accordingly, the operation of ESP at low temperatures, preferably at 140-200 \({}^{\circ}\)C, is a key procedure to reduce PCDD/Fs formation by de novo synthesis, allowing simultaneously adsorption on fly ash to decrease emissions with flue gases [127; 128; 129; 124]. Efforts have been dedicated to characterizing PCDD/Fs fingerprints, and to find congener's surrogate for TEQ estimation, which could allow faster and cheaper diagnostics of emission sources [130]. In combustion systems, PCDFs/PCDDs ratios (F/D) larger than one are interpreted as a preponderance of de novo synthesis that yields more PCDFs, while the precursor mechanism is believed to generate PCDDs preferentially [129; 131; 132]. F/D ratios have also been correlated with Cl availability; low concentrations appear to be mostly used for chlorination of phenols, forming preferentially PCDDs at high temperatures. High Cl in fuels is often associated with the worst combustion conditions, producing more PICs that are further cracked and chlorinated to form PCDFs in the post-combustion zones [133]. Therefore, the F/D ratio increases for higher Cl contents. Amounts of Cu in fuels also increase F/D ratios, due to higher PCDFs formation in the post-combustion zones [134]. On the other hand, the presence of moisture in the gas phase has been found to increase PCDDs formation over PCDFs, at low temperatures [135].