## 3 Results and Discussion
Figure 5: Changes in Cu content in the examined biomass depending on ash-forming temperature. Considering that the average Ni content in the Earth's crust amounts to 84 ppm, it was assumed that the content determined in the examined ash was not excessive (maximum 127 ppm). In combustion processes, Ni remains primarily in the slag. A characteristic feature of the behavior of nickel in the conditions of pyrolysis and gasification is the ability of this metal to form a sulfide phase and its transformation during oxidation, depending on the temperature, into the sulfate (at lower temperature) or oxide (at higher temperature). During pyrolysis without external gases, nickel sulfide Ni\({}_{3}\)S\({}_{2}\) is transformed at 757 \({}^{\circ}\)C into elemental nickel. At lower temperatures (427 \({}^{\circ}\)C), nickel is in the form of sulfides and at higher temperatures (827 \({}^{\circ}\)C) in the form of metallic sulfide. The highest Pb content was determined in pine ash, an average of 9.4 ppm, and the lowest in sunflower husk 5.1 ppm. It was clearly observed that the Pb content decreased with the increase of ash-forming temperature for all the types of biomass studied (Figure 7). Figure 6: Changes in Ni content in the examined biomass depending on ash-forming temperature. The content of this element was not excessive (maximum 13.1 ppm), while the average Pb content in the Earth's crust is 14 ppm. Pb in combustion processes goes mainly to volatile dust. As described in [20], in the case of simple pyrolysis, the dominant form at lower temperatures (up to about 527 \({}^{\circ}\)C) is solid lead sulfide (PbS). At slightly higher temperatures, this form of sulfide disappears, and in its place appears a gas form of PbS, which is accompanied by vapors of metallic lead. The significance of elemental lead in the gaseous form in relation to the gaseous sulfide form increases with increasing temperature and from about 827 \({}^{\circ}\)C the metallic lead vapor prevails over the sulfide vapors. At higher temperatures in the initial period, lead is roughly divided between the two gaseous forms of sulfide and elemental lead. After changing the potential of the system to oxidation (at 827 \({}^{\circ}\)C), a complex sequence of transformations to lead sulfate follows, which is accompanied by lead oxide PbO in the form of vapors. At an even higher temperature of 1227 \({}^{\circ}\)C, only PbO is present in the system. After changing the potential for oxidation, the basic form of lead is PbCl\({}_{2}\). After the temperature rises to about 1227 \({}^{\circ}\)C, the main form in the reduction zone is elemental lead in the form of vapors. In the oxidizing area, lead oxide in the form of vapors predominates, supplemented by two chloride forms, PbCl and PbCl\({}_{2}\). The highest Fe content was found in sunflower husk ash (on average 3681 ppm), the lowest in willow (on average 1238 ppm) and the content in this type of biomass was significantly different from the content in the other materials tested (Figure 8). Fe content increased with increasing ash-forming temperature with different dynamics depending on the raw materials. Figure 7: Changes in Pb content in the examined biomass depending on ash-forming temperature. The Fe content in the examined ash was not excessive (maximum 5797 ppm), in the Earth's crust this element occurs on average in the amount of 5.63%. Iron is usually not treated as a nuisance, let alone a dangerous element, but sometimes in conditions of its high concentration in the material subjected to thermal processes this element can potentially compete with other metals in chemical equilibria and create characteristic phases. Up to the temperature of 647 \({}^{\circ}\)C, iron oxides (Fe\({}_{3}\)O\({}_{4}\) and later FeO) will be present in the system, which should be accompanied by FeS, binding most of the sulfur contained in the starting material.