A comparative evaluation of minerals and trace elements in the ashes from lignite, coal refuse, and biomass fired power plants

Coal being a limited source of energy, extraction of energy from other sources like lignite, coal-refuse, and biomass is being attempted worldwide. The minerals and inorganic elements present in fuel feeds pose different technological and environmental concerns. Lignite ash, refuse ash, and biomass ash collected from Indian power plants burning lignite, coal-refuse, and mustard stalk, respectively, were analyzed for physico-chemical characteristics and trace elements. The lignite ash has high SiO2, CaO, MgO, Al2O3, and SO3; the refuse ash has high SiO2 and Fe2O3, but low SO3; the biomass ash has high SiO2 (but low Al2O3), and high CaO, MgO, K2O, Na2O, SO3, and P2O5. A substantial presence of chloride (2.1%) was observed in the biomass ash. Quartz is the most abundant mineral species. Other minerals are mullite, hematite, gehlenite, anhydrite, and calcite in the lignite ash; orthoclase in the refuse ash; albite, sanidine, gehlenite, anhydrite, and calcite in the biomass ash. Ashes with high concentrations (> 100 mg/kg) of trace elements are: lignite ash (V < La < Mn < Cr < Ni < Nd < Ba < Ce, Zn < Sr); refuse ash (Cr < Ce < V < Rb < Mn < Sr, Zn < Ba); biomass ash (Cu < Zn < Ba, Sr). Based on Earth crust normalization, Co, Ni, As, Se, Mo, Zn, Pb, U, and REEs (except Pr and Er) are enriched in the lignite ash; molybdenum, Zn, Cs, Pb, Th, U, La, Ce, and Lu in the refuse ash; and Mo, Zn, Sr, Cs, Pb, and Lu in the biomass ash. Elements As, Zn, Mo, Ni, Pb, Rb, Cr, V, Ba, Sr, and REEs are correlated with Al, indicating the possibilities of their association with aluminum silicates minerals. Similarly, barium, Cs, Th, and U are correlated with iron oxides; molybdenum and Sr may also be associated with sulfates and chlorides. Due to the alkaline nature of these ashes, the high concentrations of As and Se in the lignite ash; molybdenum in the biomass ash; and Se in the refuse ash may pose environmental concerns.

Research highlights
► We examined comparative mineralogical and elemental characteristics of ashes.
► Lignite, coal-refuse, biomass ashes are from PFC, FBC, grate-system, respectively.
► Major ash oxides: lignite-Si, Ca, Al, S; refuse-Si, Fe; biomass-Si, Ca, Mg, K, S, P.
► Lignite ash has highest elemental content followed by coal-refuse and biomass ash.
► Alkalinity and high-content of As, Se, and Mo may pose environmental concerns.