Composition of Oil Shale Ashes From Pulverized Firing and Circulating Fluidized-Bed Boiler in Narva Thermal Power Plants, Estonia

Abstract

The composition of oil shale ashes from pulverized firing (PF) and circulating fluidized-bed boiler combustion (CFB) systems at the Eesti and the Balti Thermal Power Plants was studied by means of quantitative X-ray diffraction, chemical analysis and scanning electron microscopy. The composition of ash varies systematically in the boiler gas pass, depending on the firing technology. PF ashes are dominated by free lime, whose content decreases from the furnace to the last fields of electrostatic precipitators. Same trends can be observed within residual fraction (i.e. mineral phases remaining in firing process) and Ca-silicates. Anhydrite, on the other hand, concentrates into the end of the gas pass. CFB ash shows more complex variations, and on several occasion the changes in certain phases are opposite to variations in respective PF ashes. Comparison of the phase composition of ashes from the same type of boilers in the Eesti and Balti TPP show remarkable differences in Ca-silicate phases. To our best knowledge, earlier mineralogical studies have not addressed compositional variations of ashes from two separate power plants. These differences are probably related to the heterogeneity of oil shale fuel, not so much to combustion temperature.

Open-Air Deposition of Estonian Oil Shale Ash: Formation, State of Art, Problems and Prospects for the Abatement of Environmental Impact

“…MgO is mostly derived from the thermal decomposition of dolomite (CaMg(CO 3 ) 2 ) and this variability is evidently due to the variation in dolomite admixture in raw OS, depending on the geological section used for mining. The separation of S along the boiler gas pass in PF ashes shows a gradual enrichment in finer ash fractions, whereas in CFBC boilers, as expected, the S proportion is the highest in the ash removed in the INTREX preheater [37,39].…”

“…Kuusik et al [37] and Bitjukova et al [39] showed that all ash types are dominated by the presence of CaO (total) whose proportion varies from 26 to 55% in the PF ash and from 26 to 49% in the CFBC ash. The proportion of CaO along the boiler gas pass decreases both in PF and CFBC boiler ashes, from 49% in the bottom ash to 30% in the last fields of the electro- static precipitators (average values).…”

“…The micromorphology of ash fractions [37,39] shows that there are considerable differences between the ashes formed in PF and CFBC boilers (Fig. 6).…”

“…As a result, anhydrite is mostly found in the first part of the boiler gas pass (in the INTREX ash) and, to a somewhat lesser degree, in adjacent zones (Figs. 4, 5) [37,39].…”

“…The content of sulphur (mainly in the form of anhydrite) is the highest in the INTREX ash of CFBC boilers. In the case of PF boilers, the sulphur content is higher in finer ash fractions [37,39].…”