1. The problem statement, all variables and given/known data Biomass with a heating value of 15 MJ/kg burns in steam generator at a temperature of 1200K and all of the heat is given to the steam at a temperature of 600K. The ambient temperature is 300K. Calculate exergy of combustion products, steam and the loss of exergy. 2. Relevant equations ex = q * [1 - (Tsurr / T)] 3. The attempt at a solution I don't really 'get' what's really going on here. Can somebody explain me why should I be using surrounding temperature to calculate exergies for both combustion products and steam? I can't make connection of the fact that burning biomass is giving heat to the steam with these equations. I guess it turns out the solution should be: ex1 = 15 MJ/kg * [1 - (300K / 1200K)] = 11.25 MJ/kg ex2 = 15 MJ/kg * [1 - (300K / 600K)] = 7.5 MJ/kg However, I don't know why. It turns out the solution to the loss of exergy is 33% (As stated in solutions) (11.25 MJ/kg - 7.5 MJ/kg) / 11.25MJ/kg = 0.33 ? But why? Can somebody explain me what is going on?