# 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
[PERSON] [27] verified that the PCDD/F contents in ESP fly ashes were higher in European boilers, burning forest biomass, than in similar USA boilers, possibly due to higher Cl levels present in the European forest biomass harvest near the Ocean's coasts. Sulphur has been broadly described as decreasing Cl activity to produce PCDD/Fs [164; 165; 166; 167; 168; 169; 170; 171; 172; 173]. At high S concentrations (S/Cl > 10), in the presence of H\({}_{2}\) O, inhibition of de novo synthesis is due to SO\({}_{2}\) oxidation to SO\({}_{3}\) reverting Cl\({}_{2}\) to HCl [174]. The same effect was found for lower S/Cl; below 2.5 [175], and also below 0.64 [166]. Another inhibition mechanism consists in the poisoning of Cu catalyst by formation of CuSO\({}_{4}\) (stable until about 800 \({}^{\circ}\)C) [165; 166; 167; 168; 176; 177; 178]. [PERSON] et al. [177] pointed out that the higher stability of CuSO\({}_{4}\) favors its retention in furnace bottom zones; thus, lower Cu contents in fly ash are available for de novo synthesis. They also suggested that sulfonation of the phenolic precursors prevents their conversion to PCDD/Fs. Therefore, the presence of S may affect the F/D ratios, given the lower formation of PCDFs [172]. [PERSON] and [PERSON] [176] proposed a positive correlation of PCDD/Fs emissions on Cl and Cu, and inverse on S for several fuels. Since natural biomass usually contains low S concentrations, the increase of S may be achieved by co-combustion with other fuels containing higher S or use of S-base additives. Other chemical inhibitors may be used [120; 155; 179; 180], such as the addition of N-S containing compounds, for example, (NH\({}_{4}\))\({}_{2}\) SO\({}_{4}\) (ChlorOut technique) and urea [181; 182; 183; 184; 185; 186]. Similarly, to Cu, other metals present in fuels, namely Fe, Ni, Cr, Zn, Pb and Al, catalyze PCDD/Fs formation, chlorination and dechlorination [187; 188; 189; 190; 110; 111; 116]. For metal chlorides [PERSON] et al. [189] consider two functions; as catalysts and as Cl donors, CuCl\({}_{2}\) being the most active, followed by CrCl\({}_{3}\), NiCl\({}_{2}\), and ZnCl\({}_{2}\). [PERSON] and [PERSON] [192] verified that CuCl and CuCl\({}_{2}\) were better as Cl donors than as catalyzers, at 353 \({}^{\circ}\)C, in the presence of O\({}_{2}\). Meanwhile, earth alkaline metals, Ca and Mg, have shown a depressor effect on PCDD/Fs formation [116; 193]. CaSO\({}_{4}\) is formed in the bottom zones of FB systems, consuming SO\({}_{3}\), which converts Cl\({}_{2}\) back to HCl. Ca may also capture HCl forming liquid CaCl\({}_{2}\) in the FB bottom zones [169], and/or absorb HCl in the post-combustion zones, preventing formation of Cl\({}_{2}\)[194]. This effect was found to be maximized at 350 \({}^{\circ}\)C [195] and 310 \({}^{\circ}\)C [196]. Moreover, NaOH and KOH have shown similar effects [197]. However, ashes are mainly composed by aluminas and aluminiosilicates, which may serve as active catalytic surfaces for PCDD/Fs formation in the post-flame zones [198]. PCDD/Fs congeners, and other PICs, results from balances of formation, destruction, and dechlorination/hydrogenation [116; 132; 145; 199; 190; 191; 192; 193; 194; 195; 196; 197; 198; 199; 199; 190; 191; 192; 193; 194; 195; 196; 197; 198; 199; 199; 190; 191; 193; 1  
involved in PCDD/Fs formation requires a more exhaustive analysis than the target list compounds normally used for risk assessment [215]. PAHs compounds are formed by complex mechanisms involving cyclization and aromatization of carbonaceous matter in the pyrolysis phase of combustion process, at temperatures typically higher than 400 \({}^{\circ}\)C, with optimum temperature in the 700-900 \({}^{\circ}\)C range [216; 217; 218]. The thermal destruction of PAHs requires higher temperatures and oxidant conditions [219; 220]. Therefore, the yield of PAHs depends on fuel composition and contamination, combustion conditions (temperature, oxygen, residence time and turbulence) and also flue gas treatment [34; 50].