## 3 Results
### Relative Enrichment Factor Elements in Ash and Deposits  
It is known from the literature that in the ash from coalburning, the concentration of trace elements increases with decreasing particle size, which is influenced by the specific surface and sorption processes [59; 82]. Similar relationships are known for major elements (Ca, P, Mg, and Si) in ash from biomass combustion but for K, the opposite has been shown [43]. The highest amounts of fine-grained particles are present in samples of WM-D deposits, followed by PB-D (Figure 6). The comparison of the mean grain size (d\({}_{050}\)) for WM-D (0.114 mm) and WM-A (0.265 mm) shows that the WM ash particles have a coarser character. PB combustion products show a similar trend. The mean particle size (d\({}_{050}\)) for PB-D is 0.122 mm, and for PB-A, it is 0.249 mm. A completely different shape of particle size distribution curves was found for deposits from CB combustion (Figure 7), where BEC3 CB-D has a mean grain value of 0.71 mm (specked particles are present), while BEC2 CB-D has a mean grain size value of 0.42 mm. The shapes of the ash grain curves are almost identical; the mean particle size for BEC2 CB-A is 0.254 mm, and for BEC3 CB-A, it is 0.287 mm. Figure 5: Trace elements (As, Cr, Cu, Mn, Ni, Pb, and Zn) with chlorine released from the burning of 1 kg of fuel. The average size of the particles that make up the deposits is significantly smaller than the ash particles except for BEC3 CB-D. Therefore, it can be assumed that there could be enrichment by both major and trace elements, even in this case. The relative enrichment factor (RE) defined by [PERSON] [83] was used to monitor the acquisition of by-products of combustion. [PERSON] [83] classified trace elements contained in coal on the basis of the relationship between the temperature and degree of volatilisation during the combustion in large energy sources (Table 10). During the combustion of PB, higher values of RE were found for ash (except S, Mn, and Cl). When burning CB, major elements Al, Si, Ti, and Fe, and selected trace elements Cr, Ni and Cu have higher values of RE in ash, while cations Ca and K and trace elements Pb, Zn, and As have higher values of RE in deposits (Figure 8). Completely different results were obtained when burning WM. All monitored elements (both major and trace) have a higher relative enrichment factor RE in deposits (Figure 9). The highest values were found for Pb and Zn. Figure 6: The grain size analysis of ash (A) and deposits (D), PB-A, PB-D and WM-A, WM-D.  
Figure 7: The grain size analysis of combustion products from coal briquettes. Some regularities resulting from the behaviour of elements during the burning of coal [83] cannot be used to identify unauthorised fuels burned in domestic boilers for solid fuels, as documented by the results obtained from the burning of paper briquettes. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **RE-BA** & **RE-FA** & **Meij [83]** & **D-CB** & **D-WM** \\ \hline Class I & ca. 1 & cca1 & non-volatile elements, equally distributed in bottom ash and fly ash & Al, Ca, Ce, Cs, Eu, Fe, Hf, K, La, Mg, Sc, Sm, Si, Sr, Th, Ti & Mn, P & Mn, P \\ \hline Class II & \(<\)0.7 & & & Ba, Cr, **Mn**, Na, & K & **Mn**, As \\  & IIc & 1.3 \(<\ldots<\) 2 & volatile in boiler but condensation on ESP & Be, Co, **Cu**, Ni & S, Ca, Cl & S, Ca, K, Fe, Ti, **Cu** \\  & IIa & \(>\)4 & & As, Cd, Ge, Mo, **Pb**, Sb, Tl, **Zn** & **Zn**, Fe & **Zn**, **Pb**, S \\ \hline Class III & \(<\)\(<\)1 & & & very volatile without condensation on ash particles & B, Br, C, Cl, F, Hg, I, N, S, Se & Cr, Ni, Cu, Pb, As & Al, P, Cr, Ni \\ \hline \hline \end{tabular} Explanation: BA—Bottom ash; FA—Emissions of fly ash; **bold—**elements in accordance with the classification of [PERSON] [83].  
\end{table} 
Table 10: Comparison of elements characterised by the RE value according to the classification of [PERSON] [83].