The performance of sugar cane bagasse boilers is commonly analyzed through the first law of thermodynamics, using the energy balance method and the fuel lower heating value as calculation base. This work presents a first law analysis using two different methods: the input/output and the energy balance. The employment of the fuel higher and lower heating values as calculation base are presented and discussed. Moreover, a second law analysis is showed, based on two methods: input/output and exergy balance. The methods based on exergy concept permits to observe the main irreversibilities that happen during the steam production in a bagasse boiler.