The Composition and Origin of PM1-2 Microspheres in High-Calcium Fly Ash from Pulverized Lignite Combustion

Abstract

This article presents the results of a systematic study on the composition and origin of PM1-2 microspheres in high-calcium fly ash. The composition of individual microspheres was studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. It is shown that the compositions of the analyzed microspheres satisfy the general dependency with a high correlation coefficient: [SiO2 + Al2O3] = 88.80 − 1.02 [CaO + Fe2O3 + MgO], r = −0.97. The formation pathway is parallel to the general trend: anorthite, gehlenite, esseneite, tricalcium aluminate, ferrigehlenite, and brownmillerite. The microspheres were classified into four groups depending on the content of major components: Group 1 (CaO > 40, SiO2 + Al2O3 ≤ 35, Fe2O3 < 23, MgO < 16 wt %); Group 2 (30 < CaO < 40, SiO2 + Al2O3 ≤ 40, Fe2O3 < 27, MgO < 21 wt %); Group 3 (CaO ≤ 30, 40 ≤ SiO2 + Al2O3 ≤ 75, Fe2O3 < 10, MgO < 10 wt %); and Group 4 (14 < CaO < 40, SiO2 + Al2O3 < 14, Fe2O3 > 30, MgO ≤ 14 wt %). A comparative analysis of the relationship between major component concentrations suggests the routes of PM1-2 formation from feldspars and Ca–, Mg–, and Fe–humate complexes during lignite combustion.