# Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers
## 3 Experimental Part
### Results and Discussion  
#### 3.2.1 Chemical Composition and PCDD/F in Biomass, Sludge and Soil  
Table 4 presents the ranges of mineral composition of the individual biomass samples collected in each boiler, plus the sludge mix. The composition of biomass samples is typical of woody biomass [231]. The wide ranges of ash concentrations and major ash forming elements, like Si and Ca, may be due to seasonal fluctuations, mainly because of higher soil adhesion to biomass in the raining seasons. Since the extraneous mineral fractions of biomass were excluded from analysis, higher quantities of these materials can be found in produced ashes than the analysis would lead on. The results obtained suggest that co-combustion of sludge might also increase ash production in the boilers, given their high contents of ash forming elements, such as Ca (23%). In addition, the sand used as a fluidizing bed agent in BFB systems also contributes to ash amounts. Scatter of other elements concentrations may reflect natural biologic cycles, although interpretation of variations can be misled due to dilution effects of major elements. In addition to Si (0.169-2.44%), Ca is the main ash-forming element of biomasses (0.21-1.83%), followed by K, Na, Al, Fe, Mg and P. Of the heavy metals measured, Mn is preponderant, as usually found in woody biomass [231]. Cu, Ni, Cr, Pb, Zn and Cd are found in trace levels, below 140 mg/kg, and Hg does not exceed 0.05 mg/kg. One biomass sample collected in BFB3 is responsible for the extreme values of Cu, Zn, Pb, and Cd levels (Table 4). Concentrations of S (0.01-0.16%) are low, as for similar biomass [231]. However, the Cl contents show a wider range, 0.04-0.28% (77% of which are above 0.15%) which is above the common Cl levels found in woody biomass (0.01-0.05%) [231]. The higher concentrations of Cl might be explained in light of the constitution of the residual forest biomass consumed in the studied boilers; mainly eucalyptus banks from pulp & paper industries. Wood barks originated from trees harvested in oceanic's coasts regions are usually more contaminated with dust deposits and salty mist coming from the sea, mainly containing NaCl [27]. Indeed, the overview of biomass constitution of [PERSON] et al. [231] refers to a eucalyptus bark having 0.26% of Cl. Other works [163; 152] report the same issue, and established correlations between Cl and PCDD/Fs formation in wood boilers. Many of the Cl contents of the residual forest biomass samples approach the guide limit, Cl < 0.3%, advised to minimize PCDD/Fs formation during combustion of cereal residues [232]. As aforementioned, high S/Cl ratios are important for inhibiting PCDD/F formation. For the biomass samples analyzed, \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Ash Stream** & **Combustion Technology** & **Ash Flows Included** \\ \hline \multirow{4}{*}{**MODEL**} & & BABFB1 & BFB & Bottom bed \\  & & BABFB3 & BFB & Bottom bed \\  & & BABFB4 & BFB & Bottom bed + EC + SH \\  & & BACF & GF & Grate ash \\  & & FABFB1 & BFB & Fly ash ESP + EC + SH \\  & & FABFB3 & BFB & Fly ash ESP + EC + SH \\  & & FABFB4 & BFB & Fly ash ESP \\  & & FAGF & Grate Furnace & Fly ash ESP + EC \\ \hline \hline \end{tabular} Notes: BFB—Bubbling Fluidized Bed, GF—Grate Furnace, SH—Superheater, EC—Economizer, ESP—Electrostatic Precipitator.  
\end{table} 
Table 3: Identification of biomass ash streams. except one case (1.3), the S/Cl ratios (0.02-1) were possibly too low for effective PCDD/Fs inhibition. Co-combustion of sludge similar to the one analyzed could increase S in the system, however given its high Cl content, the S/Cl would not be significantly improved. Table 5 presents the contents of PCDD/Fs of three biomass blends from BFB3, BFB4 and GF, plus the sludge mix and the soil sample tested. PCDD/Fs congeners are not detected in this sludge. The soil contains 43 ng/kg of OCDD, which is comparable to other forest soils in Germany [233]. Other congeners might be present, though in lower concentrations than the EQLs reported.