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This study investigates heat transfer conditions and quantifies heat losses in a 150 kW rotary kiln with passing bed material. Measurements of gas and wall temperatures, gas compositions, and radiative heat transfer were conducted for propane combustion, oxygen-enriched propane, and resistance heating. Mass and energy balance results identify air leakage, flue gas losses, and surface heat losses as key heat loss mechanisms. For propane combustion, flue gas and surface losses accounted for 29 and 38% of total energy input, respectively. Oxygen-enriched propane reduced flue gas losses to 21%, while surface losses increased to 47% due to localized heat spots. Resistance heating provided uniform temperatures, with 52% surface losses and minimal 5% flue gas losses. Scaling analysis showed reduced surface losses at industrial scales─11% for propane, 12% for oxygen-enriched combustion, and 16% for electrification, while flue gas losses were 43, 19, and 5%, respectively. Energy transfer efficiency for calcination was quantified at 45% for propane and 60% for electrification. This work establishes a validated framework for measuring, quantifying, and scaling heat losses in rotary kilns.