## 3 Results and Discussion  
The ash content found in the examined biomass was generally dependent on the biomass type and ash-forming temperature (Figure 1). The lowest ash content was found for oak and it ranged from 0.29% at the temperature of 1000 \({}^{\circ}\)C to 2.21% at 500 \({}^{\circ}\)C, while the highest ash content was found for wheat straw, from 4.08% at 1000 \({}^{\circ}\)C to 6.16 % at 500 \({}^{\circ}\)C.  
Figure 1: Changes in ash content in the examined biomass depending on ash-forming temperature. The obtained test results indicate that the compactness of ash in the studied biomass was relatively low and typical for a given raw material [18; 19], regardless of ash-forming temperature. In wood, ash accounted for less than 2%, whereas in agricultural crops it could amount from 5-10% to as much as 30-40% [18]. The ash content in biomass depends on the sampling site, harvesting time and harvest conditions. Thus, for example, its content in wood and woody biomass was below 1%, in energy crops from 1.79% to 5.38%, in agricultural biomass from 6.88% to 9.2%, in forest residue from 6.27% to 10.44% and in agri-food industry wastes from 1.06% to 9.98%. The changes in the ash content in the examined biomass depending on ash-forming temperature (Figure 1) clearly indicated that it dropped with increasing temperature, with the largest differences for ash from wood biomass (above 70-80%). As the temperature increased, the percentage of content drop went lower for ash from miscant and wheat straw (34%). This is the result of the volatilization of most of the components contained in the biomass to the gas phase during incination. The observed differences may be related to the higher content of mineral compounds in the biomass of grass plants. The content of heavy metals in the studied biomass varied within very wide limits. Differences between particular types of organic material, even of the same type, were of the order of magnitude. The results of the determinations are collected in together with statistical analysis of the results are collected in Tables 1-4 together with statistical analysis of the results. The highest Cr content was found in sunflower husk ash, on average 67.8 ppm, obtained at all considered temperatures, except for the temperature of 1000 \({}^{\circ}\)C, for which the highest Cr content was found in willow ash (97.1 ppm) (Figure 2). The lowest content of this element was found in miscant ash, on average 34.1 ppm. In all studied biomass types, the content of Cr in ash increased with ash-forming temperature, but with different dynamics--the interaction effect occurred, among others, for willow, oak and pine, where, for samples incinated at 1000 \({}^{\circ}\)C, a significant increase in Cr was observed. Figure 2: Changes in Cr content in the examined biomass depending on ash-forming temperature. Considering that the average content of Cr in the Earth's crust is 112 ppm, the content of this element was not high. Cr is a metal that in combustion (oxidation) technologies remains in condensed (solid) phases, regardless of the oxygen potential of the system and the presence of sulfur and chlorine. Both in the pyrolysis and gasification processes under Cr reducing conditions, it takes the form of Cr\({}_{2}\)O\({}_{3}\) oxide up to 1067 \({}^{\circ}\)C. According to [PERSON] [20], regardless of the form in which Cr isin the original material, this element should ultimately remain in solid post-process residue and in insoluble form. The above was confirmed by the obtained test results. The highest Zn content was found in willow ash, an average of 601.4 ppm, and the lowest in oak ashes, an average of 114.9 ppm. As ash-forming temperature increased, the content of this element in the ashes decreased, a similar dynamics of Zn changes was observed for willow and miscant (Figure 3). The determined Zn content in the examined ash differed significantly from the average content in the Earth's crust, which is 70 ppm. In oxidation technologies, Zn mainly moves to the gas phase.