## 3 Results and Discussion
### Chemical Composition of Raw Materials  
The waste materials have not undergone any chemical treatment as part of the manufacturing process and, thus, are natural products in this respect. In addition, since they are materials that are used for direct consumption, their processing is subject to higher demands in terms of quality, low residue content, limit of heavy metal concentration and compliance with limits on the content of other harmful substances. Thus, their use for energy generation is unlikely to pose a problem in terms of meeting the limits or restrictions for environmental and public health reasons [13]. However, it should be also kept in mind that the restrictions and thus controls are for the edible parts of the fruit, and waste can still have above-limit contaminants. A number of authors point out that the accumulation of substances can vary significantly according to the plant part and the tissue that is burned [12; 51]. For example, elevated concentrations of foreign or harmful substances are reported in leaves or seeds and are often related to contaminations present in the environment and the use of chemicals during cultivation. These waste products have been evaluated in other regions as well, and the results are similar in terms of energy and content [40; 41; 43; 52; 53; 54]. Even comparisons with materials exotic to Europe or with vegetable residues give similar results [42; 55; 56; 57]. [PERSON] et al. [35] report very similar C, O, H and S compositions for grape pomace and grape seeds to ours. [PERSON] et al. [40] report similar results for berry pomace for C and H to ours, while reporting 43% for O, compared to our value of 33%. Conversely, for S they report a significantly lower content of only 0.1%. [PERSON] and [PERSON] [41] report very similar results for C, O, and H for peach stones, but again, a slightly lower S content. Elevated concentrations of chlorine, sulphur or copper, which probably originate from agrochemicals, as well as iron, calcium and nitrogen, which probably originate from fertilizers, can be observed. Monitoring the chemical composition is all the more important, as the burned waste materials may not reach normal combustion temperatures or the combustion process may be non-standard, resulting in increased emissions to the environment, or condensates in the boilers, or increased concentrations in the ash. The resulting energy generation may, therefore, not only be limited by lower calorific value of the fuel used, but also by the need to respond to the limits of contaminants, with utilization of different filters or modifications to the combustion equipment, or increased requirements for ash disposal [5; 38]. In the samples tested, no elevated concentrations of the elements (As, Cr, Cu, Pb, Hg, Ni and Zn) exceeding the ISO 17225-6 standard were detected, and all materials meet the standard in this respect (Tables 3 and 4). In the samples tested, no elevated concentrations of the elements (As, Cr, Cu, Pb, Hg, Ni and Zn) exceeding the ISO 17225-6 standard were detected and all materials meet the standard in this respect (Tables 3 and 4). A significant result of the analyses is the mineral composition of the samples, with some differences in the specific elements such as K, S, Fe, Ca, Cl, Al, P and Si. Differences in the content of individual metals are another reason for the higher ash content [58]. The mineral matter of biomass in combination with organic composition, moisture content, etc. plays a major role in determining the properties of the resulting fuel [51]. According to the results of the analyses, it is clear that similar materials, such as polaces or stones, do not have similar mineral composition. This also confirms the results of a number of authors who report that different parts of the fruit can have diametrically different compositions [18; 59]; in our case, for example, grape vine pomace and grape vine seed cake, or almond seed coat and shell.