With coal being the predominant and limited source of energy, extraction of energy from lignite, coal-washery rejects, and agro-wastes through combustion is being attempted globally. During combustion, minerals in the fuels undergo various transformations with corresponding impact on the process. Lignite fly ash, Jamadoba fly ash, and Kalptaru fly ash and corresponding fuels like lignite, coal-washery rejects, and mustard stalk from pulverized fuel, fluidized bed, and grate system of combustion at Neyveli, Jamadoba, Kalpataru, respectively, were characterized. Lignite has better fuel characteristics than washery rejects and mustard stalk. Of the major phases, SiO2, Al2O3, and CaO are common, besides Fe2O3 in lignite fly ash and Jamadoba fly ash, and K2O in Kalptaru fly ash. Among minor phases, MgO, Na2O, SO3, P2O5, and TiO2 are common, despite K2O in lignite fly ash and Jamadoba fly ash, and Fe2O3 in Kalptaru fly ash. Other minerals are mullite, silicates, sulphates, and carbonates of Ca in lignite fly ash; K-Al-Si species in Jamadoba fly ash; K/Ca-Al-Si species, and sulphates and carbonates of Ca in Kalptaru fly ash. These minerals are the product of mineralogical transformations, where source and composition of the fuels and operating conditions are crucial. Overall the combustion of lignite is more successful than that of washery rejects and mustard.