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List of symbolsA Area (m 2 ) C Carbon content (%) c Specific heat (kJ/kg K) h Enthalpy (kJ/kg) H Heat transfer coefficient (kW/m 2 °C) HV Heat value HHV Higher heating value (kJ/kg) L Energy loss (%) LHV Lower heating value (kJ/kg) m Proportion of material flow per kilogramme of fuel (kg/kg) m Mass flow rate (kg/s) Q Available heat (kJ/kg) Q Heat flow (kW) T Temperature (°C) U Uncertainty x Mass fraction (kg/kg) Greek letters λ Water vaporisation heat (kJ/kg) η First law efficiency Subscripts ash Ashes ah Drum ashes amb Ambient b Bagasse bd Blowdown in the boiler conv Convection C, CO, Co 2 Carbon, carbon monoxide, carbon dioxide db Dry bagasse Abstract This work presents an experimental efficiency analysis of sugar cane bagasse boilers performed according to the first law of thermodynamics. The methodology employed to determine efficiency regarding the first law is the indirect or energy losses method, based on the energy balance method established in the most recent standards, ASME PTC 4-2008 and EN 12952-15:2003. Experimental data were obtained from boilers installed in sugar and ethanol mills in the state of São Paulo, Brazil. Calculation of heat losses through the boiler walls is presented in detail and compared with a previous calculation method recommended in an older standard (ASME PTC 4.1 1964). Moreover, calculation of the uncertainty of the efficiency determination is presented and discussed. On the basis of the obtained results, the main causes of energy losses are identified, and suggestions aimed at improving boiler efficiency are made.