An analysis of equilibrium phase formation in the ashes from Polish coal and cedar nut shells (CNS) was performed for different combustion conditions in a bubbling fluidized bed boiler. Equilibrium calculations were performed using Factsage 6.3 for five excess air ratio values (lambda)-0.7, 0.9, 1.0, 1.2, and 1.6-at a temperature range of 650 <= T <= 1050 degrees C. The masses and compositions of ashes synergistically depend on lambda and T. An increased combustion temperature results in a decrease in the mass of (1) ashes formed from coal and CNS and (ii) liquid slag (in the case of CNS), resulting from a lower content of K2CO3(1) (the lower the lambda, the greater the decrease in K2CO3(1)), and the migration of potassium into the exhaust gas in the form of compounds KCl(g), KOH(g), and K2SO4(g). Ashes from both coal and CNS contain compounds present (i) throughout the whole temperature range, regardless of lambda value, e.g., KAlSi2O6(s) and Mg2Al4Si5O18(s), in the case of coal; e.g., Mg2SiO4(s) and MgO(s), in the case of CNS; (ii) only in some temperature ranges, e.g., Al2SiO5(s) (T < 780 degrees C) and Al6Si2O13(s) (T > 780 degrees C), in the case of coal; e.g., Ca-5(PO4)(3)(OH)(s) (T < 900 degrees C) and Ca-3(PO4)(2)(s) (T < 900 degrees C), in the case of CNS; and (iii) depending on lambda, e.g., FeS(s), Fe2Al4Si5O18(s) (lambda <= 1.0), Fe2TiO5(s), and CaSO4(s) (lambda > 1.0), in the case of coal; e.g., FeO(s) (lambda < 1.0) and Mg2Fe2O4(s) (lambda >= 1.0), in the case of CNS.