# 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
All PCDD/Fs congeners can be found in solid discharges from incinerators [136; 137; 138; 139], in increasing concentrations from the furnace bottom to the flue gas exit (bottom-ash \(<\) boiler-ash \(<\) ESPash \(<\) filter-cake-ash \(<\) fly-dust); PCDD/Fs contents in wood fly ashes were reported to be 10 times higher than in corresponding bottom ashes [140]. The PCDDs concentrations generally increase with the increasing degree of chlorination (nCl) of the congeners; OCDD is usually present in higher amounts, followed by 1,2,3,4,6,7,8-HpCDD. A more random distribution occurs for PCDFs; the hepta, hexa and penta PCDFs, being more prevalent than OCDF [141; 142; 143; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178]. Moreover, in MSWI bottom ashes lower PCD/Fs concentrations were found in larger particles [149; 150], which suggest that ash separation may provide less contaminated fractions. Cl is the limiting reactant for PCDD/Fs formation; however, the relation between PCDD/Fs yields and Cl amounts (and its speciation) is not completely understood [151; 152; 153]. Variations within low Cl contents appear to make little difference, while high concentrations strongly increase PCDD/Fs [153; 87; 154]. During combustion, Cl is released from the inorganic or organic matrix, generating predominantly HCl, although metal chlorides might also form. HCl can be converted to gaseous Cl\({}_{2}\) through the catalyzed Deacon reaction, which is maximized at about 300 \({}^{\circ}\)C, in the presence of O\({}_{2}\) and Cu. Cl\({}_{2}\) is believed to be more active as a chlorinating agent than HCl, owing to the faster production of highly reactive Cl radicals [155; 156; 157; 158; 151]. Simultaneously, metal chlorides eventually present in fly ashes, such as CuCl\({}_{2}\), might behave as Cl donors, chlorinating the carbon structures directly, in the presence of O\({}_{2}\)[118; 119; 151]. Moreover, the chlorination of dibenzofuran (DF) has been found to be favored over chlorination of dibenzo-p-dioxin (DD), corroborating that higher Cl correlates with higher F/D ratios in waste incinerators [159]. Woody biomass with low Cl contents usually produces low PCDD/Fs, while co-combustion with streams containing Cl and metals (salty wood biomass, bleached kraft mill waste water sludges, straw and cereal residues, treated wood with PCP base preservatives or other metal base preservatives, sewage sludge, etc. ), enhances PCDD/F production [160; 161; 162; 163; 164; 165; 160; 161; 27]. In the past, biomass co-combustion with paper bleaching sludge generated cyclone fly ashes with PCDD/Fs concentrations attaining 210 ng TEQ/kg [162]. [PERSON] et al. [115] verified that the firing of waste wood, impregnated with Cu base preservatives, increased the basal PCDD/Fs levels of clean wood ash by two to three orders of magnitude and the F/D increased because of preferential formation of PCDFs. [PERSON] and [PERSON] [140] also revealed that TEQs in filter ash from waste wood incineration may be three orders of magnitude higher (100-10,000 ng TEQ/kg) than for clean wood combustion. Studies performed in waste wood chips incinerators in Taiwan have shown that PCDD/Fs in baghouse fly ashes were extremely high; 38,060 ng I-TEQ/kg in the boiler operating at 850-925 \({}^{\circ}\)C and 98,570 ng I-TEQ/kg in the boiler operating at lower temperatures, 500-850 \({}^{\circ}\)C [146]. [PERSON] et al. [163] reported that combustion of salt-laden wood with 0.1-2% NaCl, in coastal boilers, resulted in PCDD/Fs emissions two to three orders of magnitude higher than those burning wood with lower Cl. More research in coastal boilers, burning salty wood, revealed that the decrease of PCDD/Fs concentrations in ESP fly ashes was influenced not only by the decrease of Cl concentrations, but was also due to working at lower furnace loads, 65%, that might have promoted better combustion [152].