## 3 Results and Discussion
Since, after the oxidation of metallic iron, the amount of sulfide phase is systematically mixed in a short time and it is also depleted and only the form of Fe\({}_{3}\)O\({}_{4}\) remains in the system, after the oxygen potential's jump, this form oxidizes to Fe\({}_{2}\)O\({}_{3}\)[20]. In combustion processes, 90-97% Fe is retained in the slag. This image is completely consistent with the results obtained; the metal remained in condensed phases and did not move to the gas phase. Thermodynamic calculations suggest that Cd and Pb are preferentially converted during combustion into CdCl\({}_{2}\)/PbCl\({}_{2}\). In addition, not only metal, but also hydrochlorides and limited metal oxides may evaporate [21]. Zn is relatively less volatile than Pb and Cd but ZnCl\({}_{2}\) and Zn metal residuals were volatilized. However, conversion to ZnCl\({}_{2}\) is limited because a significant amount of Zn is present in the form of a stable oxide. Other volatile metals, including Ar and Sb, show similar behavior and their ash content is generally very low. Hg almost completely passes into the exhaust gas due to the high vapor pressure, but the Hg content in the unprocessed biofuels is very low (0.01-1 ppm) [22; 23]. Litophilic metals, such as Fe, Cu, Cr and Al, go to coarse volatile ash in large quantities not only because of their low volatility, but also because they are in oxide form and are very stable [24]. In the literature, little research concerns the emissions of heavy metals during the small-scale combustion of various solid biofuels. Scientific reports show that, as ash-forming temperature increases,  
Figure 8: Changes in Fe content in the examined biomass depending on ash-forming temperature. the ash content decreases, which is also confirmed by the results obtained. The temperature of biomass incineration also has some influence on the migration of elements. Its increase promotes further degradation of minerals. According to [PERSON] et al. [25], Fe shows an upward trend due to its low volatility, while other metals show slight fluctuations. The work in [26] distinguishes, depending on the temperature range and biomass examined, elements of high volatility (Hg and Se), varied volatility (Cd and Pb), average volatility (Cu, Zn, Ni, and Cr) and low volatility (Mn, Ba, Co, and As). A similar division was made in [27], where trace elements are ranked according to their average retention in combustion ash. The most volatile are Hg and Se, followed by Cd, Tl, Pb and As. Partial retention is observed in Zn, Sb, Cr, V, Cu, Sr, Mo, Mn and Ba ashes. The highest retention of elements in the ash is recorded for Be, Co and Ni. The above was confirmed in most cases by the research results obtained here. Among the analyzed elements, the content of Zn, Cd, Cu, and Pb decreased with increasing temperature, while the content of Ni, Cr, and Fe increased. Thus, the exceptions were Ni Cr and Fe, whose ash content clearly increased with increasing ash-forming temperature. According to [PERSON] [28], olive oil ash was enriched with Cr, Cu, Ni and Mn, while the content of toxic elements Se and Pb was below 9 ppm. The highest Cr content at 2000 ppm was attributed to the type of fuel. The biomass biofuels examined by the authors of [29], relative to coal and peat, produced higher Zn emissions, but lower or similar emissions of the sum of other metals. In addition, Zn and Pb were the two heavy metals with the highest content in the volatile fraction. The above was reflected in the obtained test results; Zn and Pb content decreased with increasing temperature, which indicates their transition to the gas phase. The increase in temperature during pyrolysis of sewage sludge leads to the transformation of less stable heavy metals to more stable forms in the biocarbon. The degree of conversion and trends are individual for each metal and for particular materials tested [30]. [PERSON] et al.