## 3 Results
### Release of Elements into the Atmosphere during Combustion
Metal ions may also be organically associated with biomass fibres [52] and can only be released into the air when they burn out. The higher amount of Ca released from the BEC2 boiler during WM combustion is related to the degradation of calcium oxalate crystals [66] and the subsequent reaction of Ca ions with CO\({}_{2}\) to form calcite at >479 \({}^{\circ}\)C. Calcite decomposes at a temperature higher than 750 \({}^{\circ}\)C [67] for the creation of CaO. CaO has not been identified in ash and deposit products from BEC2 and BEC3 by the X-ray-diffraction method. The mineralogical analysis of ash shows that in the case of ash and deposits from BEC3, a greater amount of Ca is bound in an amorphous phase, which was formed at a higher temperature than was present in the BEC2 boiler. A surplus of Ca ions was released into the atmosphere. Wood preservatives containing As (arsenate) break down in the temperature range from 750-800 \({}^{\circ}\)C during de-arsenation [68]. The amount of sulfur in the BEC2 and BEC3 boiler emissions is affected by the thermal degradation of thioxanthene, which has been identified in the deposit products and in the input material by the Py-GC/MS method. Thioxanthene is a pyrolysis product formed by the thermal degradation of the thiophenol/formaldehyde resins [69] contained in WM. In addition to thioxanthene, other sulfur-containing organic compounds have been identified in the deposits from BEC2 from WM combustion--thiourea, tetramethyl thiourea, 3 methyl thiophene and dihydro 2,3H thiophenone, which are also produced during the thermal degradation of thiophenol/formaldehyde resins. For example, thiourea starts to isomerize to ammonium thiocyanate (NH\({}_{4}\)SCN) at temperatures ranging from 140 to 180 \({}^{\circ}\)C [70]. The absence of these compounds in deposits from BEC3 explains the degradation of sulfur-containing organic compounds in the BEC3 boiler, and hence the higher sulfur emissions. Results of the mineralogical analysis were also used to identify the differences between element release during the combustion of CB in BEC2 and BEC3. The results indicate that the amounts of maghemite in deposits from BEC2 (13.45%) and BEC3 (20.45%) vary significantly. In addition, hematite is also present in BEC3 (8.68%). Maghemite levels in ash from the combustion in both BEC2 and BEC3 are comparable (14.55 and 16.31%). Generally, higher amounts of Fe are bound in deposits from BEC3 in mineral Fe-phases than in BEC2. During combustion, iron distribution into individual phases--hematite, magnetite and Fe-glass phases--is affected by the O\({}_{2}\)/CO\({}_{2}\) ratio [71]. With a higher O\({}_{2}\)/CO\({}_{2}\) concentration, the proportion of magnetite increases and hematite decreases, while the amount of Fe-glass phase does not change. The amount of unburned carbon affects oxidation processes involving the formation of magnetite, maghemite, and hematite. After the complete burning of carbon, the magnetite present in the ash becomes completely oxidised to hematite [72]. Part of hematite and magnetite (approximately 8% of Fe) crystallises from the molten glass phases during combustion and is not significantly affected by changing combustion conditions [71]. Some trace elements are bonded to newly formed Fe-phases when coal is burned. For the deposits from BEC2 and BEC3, a polynomial dependence between Fe and Cu concentration was confirmed (r = 0.84, \(n\) = 9, \(\alpha\) = 0.005). Adsorption of heavy metals on the surface of iron oxides (mainly magnetite) in preferential order of Pb, Zn, Cu, Cr, and Cd or their substitution in minerals with spinel structure is known [73]. In the case of Zn, correlation between Zn and Ca has been demonstrated (r = 0.81, \(n\) = 9, \(\alpha\) = 0.01). Differences in the behaviour of both major and trace elements during the combustion in BEC2 and BEC3 are primarily due to the presence of elements in the input fuel and the decomposition ability of phases in which both major and trace elements are bound and influenced by temperature and oxidation-reduction conditions.