## 3 Results and Discussion
Zn is a relatively volatile metal (boiling point 907 \({}^{\circ}\)C) but quite reactive and showing high affinity to sulfur. At low temperatures, Zn should not move into the gas phase as vapors. At high temperatures (827 \({}^{\circ}\)C), Zn, in the conditions of pyrolysis, steam gasification, or initial phases of gasification with air, is reduced to metal vapors and in this form it can be discharged from the reaction zone with the gas phase [20]. This pattern of Zn's behavior is consistent with the obtained test results. Nevertheless, the behavior of zinc in thermal processes can be significantly different, depending on the type of process, temperature and composition of the biomass used. The highest Cd content was found in sunflower husk ash, on average 5.9 ppm, and the lowest in wheat straw, on average 4.3 ppm (Figure 4). In all cases, a decrease in the Cd content in ashes was observed as ash-forming temperature increased. A similar dynamics of changes in sunflower husk and miscant was observed. Figure 3: Changes in Zn content in the examined biomass depending on ash-forming temperature. The determined Cd contents in the ashes were significant (maximum 10.2 ppm), the average Cd content in the Earth's crust is 0.15 ppm. In the range of low oxygen potentials, cadmium is present in the form of solid sulfide up to the temperature of approximately 427 \({}^{\circ}\)C and then moves to the gas phase as elemental cadmium. The total passage of cadmium to the gas phase occurs at temperatures exceeding 527 \({}^{\circ}\)C. Above the mentioned temperature range, i.e., 427-527 \({}^{\circ}\)C, cadmium moves to the gas phase regardless of the oxygen potential of the system [20]. The highest Cu content was found in sunflower husk ash, an average of 284.5 ppm, and the lowest in miscant and wheat straw ashes, on average 56.1 and 54.4 ppm, respectively. As ash-forming temperature increased, Cu content decreased in the examined ash (Figure 5). Apparently, this tendency was visible for sunflower husk and oak ashes, where the changes proceeded with similar dynamics; however, in the case of miscant and wheat straw, it was almost imperceptible. The marked content of Cu in the ashes of sunflower, oak, pine and willow husks is higher than the average content in the Earth's crust, which is 60 ppm. Figure 4: Changes in Cd content in the examined biomass depending on ash-forming temperature. Copper, considered to be a precious metal, is easily reduced in favorable conditions to free metal. In combustion technologies, Cu is first retained in slag phases. However, it is characterized by a clear affinity to sulfur, which is why, in the presence of this element, in the reducing conditions, a stable combination is copper sulfide Cu\({}_{2}\)S. When oxygen potential is changed, oxidizing copper sulfide is oxidized to sulfate at lower temperatures (427 \({}^{\circ}\)C) or to copper oxide CuO at higher temperatures (827 \({}^{\circ}\)C). Under reducing conditions, when copper is bound to sulfide, small amounts of copper can pass to the gas phase in the form of CuCl, especially at temperatures higher than 827 \({}^{\circ}\)C. After changing the oxygen potential, the amount and differentiation of the oxide forms of copper in the gas phase clearly increases, which means that this metal is completely discharged into the gas phase [20]. The obtained test results show that the decrease in Cu content in ashes along with the increase in the ash-forming temperature was related to the transition of this element to the gas phase, but not to the same extent for all the biomass types tested. The highest Ni content was found in willow ash, on average 97.9 ppm, and the lowest in miscant ash, on average 18.9 ppm. This content was very different and it varied in many ways as ash-forming temperature increased. In general, the Ni content increased with the increase of ash-forming temperature with similar dynamics as for willow and sunflower husk. Only for miscant this trend was very poorly visible (Figure 6).