RESUMO
The crystallization in glasses is a paradoxical phenomenon and scarcely investigated. This work explores the non-isothermal crystallization of a multicomponent alumino-borosilicate glass via in situ high-energy synchrotron X-ray diffraction, atomic pair distribution function, and Raman spectroscopy. Results depict the crystallization sequence as Ca3Al2O6 and CaSiO4 followed by LiAlO2 with the final compound formation of Ca3B2O6. These precipitations occur in a narrow temperature range and overlap, resulting in a single exothermic peak in the differential scanning calorimetry thermogram. The concurrent nucleation of Ca3Al2O6 and CaSiO4 is intermediated by their corresponding hydrates, which have dominantly short-range order. Moreover, the crystallization of LiAlO2 and Ca3B2O6 is strongly linked with the changes of structural units during the incubation stage in non-isothermal heating. These findings clarify the crystallization of multicomponent glass, which have been inferred from ex situ reports but never evidenced via in situ studies.
RESUMO
During high temperature processes in the furnace volatile and semi-volatile elements and radionuclides are partially emitted to the environment, depending on their chemical form in the original fuel, the technological set-up of the combustion system, and the prevailing combustion conditions. Two of the world's largest oil shale-fired power plants (PPs) have been operational in Estonia from the 1960s, during which time creation of significant environmental emissions and waste containing naturally occurring radionuclides has occurred. Pb-210 and 210Po are considered natural radionuclides with the highest emission rates from PPs and possess elevated potential radiation exposure risks to humans and the environment. These radionuclides have the highest activity concentration values in fine ash fractions, especially in fractions remaining below 2.5 µm. To determine the activity concentrations of 210Pb and 210Po in the PPs' outlet, sampling was conducted from boilers operating on pulverized fuel (PF) technology with novel integrated desulphurization (NID) system and bag filters as well as with electrostatic precipitators (ESPs). The 210Pb and 210Po activity concentrations remained around 300 Bq kg-1 for the NID system compared to 60-80 Bq kg-1 in the ESP system. The dominant ash fraction in both systems was PM2.5, constituting over 50% of the fly ash mass collected from the outlet. The authors estimate that the total atmospherically emitted activity for the modernized PPs remains dominantly below 1% of the activity that is inserted via fuel. The implementation of higher efficiency purifications systems has significantly reduced the negative effect of these PPs. Based on annually emitted fly ash and boilers' working hours, the 210Pb and 210Po activity released relative to energy production were up to 68.3 kBq GWhel-1 for 210Pb and 64.6 kBq GWhel-1 for 210Po. These values are 1 to 2 orders of magnitude lower compared to the situation in the 1980s. These findings represent the first publicly available quantitative results estimating the 210Po emissions from large oil shale-fired PPs.
