###### Abstract  
This article presents the influence of ash-forming temperature on ash content and the content of selected heavy metals. The biomasses most commonly used in low-power heating boilers, such as miscant, oak, pine, sunflower husk, wheat straw and willow, were selected for the research. The biomass was incinerated at temperatures of 600 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, 800 \({}^{\circ}\)C, 900 \({}^{\circ}\)C and 1000 \({}^{\circ}\)C, using the X-ray Fluorescence Spectrometer (XRF) for the measurement of element content. The results show that the content of heavy metals in the examined ash was not excessive and could not be considered as potentially dangerous. As the ash-forming temperature increased, the content of Zn, Cd, Cu and Pb decreased, which indicates that, at higher temperatures, they went into the gas phase. Cr, Ni and Fe were thermally stable and less volatile, thus the ashes were enriched with them as the ash-forming temperature increased. biomass ash; elements; ash-forming temperature +
Footnote †: journal: Applied Soft  
costs for users [6]. Ash from biomass incineration, however, has many potential applications: in forest ecosystems, for agricultural and horticultural purposes as a fertilizer [8], as a building material [9; 10] or as an ingredient in the process of building materials production. It is particularly attractive to use as a fertilizer due to the high content of valuable macro- and micronutrients. The composition of ash from biomass is very diverse and depends to a large extent on the type of plant, its origin, incineration system, process parameters and storage conditions. Another problem that may limit the use of ashes as fertilizers is the presence of trace elements in the ash [11; 12]. The opinion that biomass is relatively low in hazardous elements is not supported by detailed analyses. The concentration of toxic (or potentially toxic) trace elements is usually lower in biomass than in coal, with the exception of Zn, Mn and Cu. However, the toxic elements contained in the biomass are more mobile during incineration compared to coal and are also more susceptible to leaching from the ash [13]. In the case of biomass grown on contaminated sites, the content of various heavy metal elements in ash may be very high. According to [PERSON] et al. [14], ash from plants cultivated in contaminated mining areas should be treated as toxic waste. In addition, biomass usually exhibits higher chlorine concentrations, which, according to the thermodynamic equilibrium calculations, are believed to particularly improve the mobilization of a number of metals from fuel particles, particularly under reducing conditions. At the same time, it should be borne in mind that the behavior of trace elements, i.e., volatilization or retention in the ash during incineration, is quite a complex problem. It will depend on the mode of occurrence of trace elements in the materials, physicochemical interactions between the components as well as the ash-forming temperature. The chemical degradation of biomass during incineration, i.e., pyrolysis, leads to the formation of a solid residue in the form of ash. Bottom and volatile ash are usually two types of ash produced and may differ in properties due to different types of biomass burned, operating conditions and type of system used. High ash content significantly reduces the energy efficiency obtained from a specific source of biomass. The ash-forming temperature and speed are influenced by the ash properties and composition [15; 16]. The aim of the work was to assess the impact of biomass type and ash-forming temperature on ash and selected heavy metals contents in view of sustainable management of biomass ashes through their use for fertilizing purposes.