This work presents the combustion characteristics of coal, petroleum coke (PC), rice straw (RS), mustard cake (MC), and their blends to assess the applicability of blended fuel for thermal power generation. Characterization results show that PC has the highest gross calorific value (GCV) (35,990 kJ/kg) to improve the overall energy density of the blend significantly. Higher volatile matter (VM) present in RS and MC improved the ignition behavior and combustion efficiency of the blend. For 10 °C/min heating rate at 350 °C, with the increase in RS in blends from 10% to 30%, combustion efficiency increased from 12.85% to 32.66%. Synergistic analysis signifies that higher biomass content enhances blends’ combustion characteristics through catalytic effects of alkali oxides present in RS/MC. Thermodynamic analysis (ΔH, ΔG, and ΔS) inferred that RS and MC combustion is easier than coal and PC. With the increase in MC in blends from 10% to 30%, ΔH decreased from 114.81 to 82.31 kJ/mol, ΔG declined from 159.33 to 122.86 kJ/mol, and ΔS improved from −63.59 to −58.14 J/mol · K, indicating blending of biomass improved the combustion.