## 3 Results
### The Form of Occurrence of Elements in Fuels
#### 3.2.3 Elements in Coal  
Elements in coal occur in different forms and concentrations depending on genetic conditions, most of the major elements being discrete minerals [55]. The presence of trace elements is linked to the degree of coalification [56]. Trace elements in coal may be present in many different forms; for example, they may occur in silicates, sulfides, carbonates, oxides, sulfates, phosphates, selenides, and halides, or they may be bound to the surfaces  
\begin{table}
\begin{tabular}{l c c c c c c c c c c c c c c} \hline \hline
**Wood** & **Al** & **Si** & **Ca** & **Fe** & **Ti** & **P** & **S** & **K** & **Cl** & **Mn** & **Cr** & **Ni** & **Cu** & **Zn** & **Pb** & **As** \\ \hline  & & & & **(\%)** & & & & & & & & **mg/kg** & & \\ \hline WM & 0.32 & 0.81 & 1.51 & 0.22 & 0.02 & 0.77 & 0.03 & 0.21 & 68.4 & 300 & 4 & 7 & 20 & 200 & 8 & 2.4 \\ Wood * & 0.003 & 0.093 & 0.75 & 0.0005 & & BDL & 0.015 & 0.02 & 114 & 100 & & & & & \\ Bark * & 0.009 & 0.02 & 0.83 & 0.009 & & 0.045 & 0.036 & 0.21 & 260 & 714 & & & & & \\ STW * & & & & & & & & & & & 60 & 3.5 & 59 & 440 & 53 \\
** & & & & & & & & & & & & 4300-9100 & & 2800-5500 & & & 3100-7600 \\
** & & & & & & & & & & & & & & \\ \hline \hline \end{tabular} Explanation: * [PERSON] et al. [52], ** [PERSON] et al. [49]; *** [PERSON] et al. [51]; CCA—Chromated copper arsenate; STW—surface-treated wood.  
\end{table}
Table 5: Content of inorganic elements in WM, wood and wood treated with preservative (literature data).  
of minerals or the organic matter. [PERSON] [57] already identified elements primarily bound to the inorganic component (As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, and Zn) and elements bound to the organic component (B, Be, Ga, Ge, P, Sb, Se, and V). According to their organic or inorganic affinity, the different division of elements was developed by [PERSON] et al. [58]. They distinguish the occurrence in mineral phases for Zn, Cd, Mn, and Pb, while the link to the organic component of coal (organometallic complexes, chelates or ion-exchange compounds) is typical for Co, Cr, and Cu. Organic compounds bound in lignite from the total content: 60% of Mg and Na, 50% of V, 35% of Al, Ca and Co, 30% of As and Ni, 25% of K, 20% of Cu, 15% of P, Ti and Zn, 10% of Cr and Mn, and 5% of Fe [56].  
It is clear from Table 6 that the contents of the major elements correspond to the published concentrations. Only the measured concentration of Ca is significantly below the value reported under the certificate of Free Company and slightly below the value reported by [PERSON] et al. [43]. A significant difference can also be seen in potassium. The measured concentration in CB corresponds to the data of [PERSON] et al. [43], but it is about four times higher than the value reported by Free Company. Higher values in CB than those reported for coal lignite Clarke [59] were found for Mn and Cr, while other trace elements, including As, have lower values.  
The highest levels of major and trace elements Al, Si, Fe, Ti, Cr, and Ni were found in CB > PB >WM. The highest levels for Ca, K, Mn, Cu, and Pb were found in PB > WM > CB, while for P and Zn elements, the highest levels were found in WM > PB > CB.