Moisture exergy

Using the molar fraction of water x h20. the thermomechanical exergy is split into the dry gas exergy ei, ,tm,dry and moisture exergy e , ,tm,H20 both in kj/kmol, the latter referring to the condensed state of water. 

The mean specific molar heat capacity of the dry gas mix Cpm.ilT l, can be easily calculated or extracted as a stream property from a process simulator, for example, Aspen Plus [2]. The moisture exergy ei,H,tni,H20 must include the complete condensation of any water vapor. Although the specific enthalpy ho = 1890.3 kJ/ kmol and entropy So = 6.616 kJ/(lcmol K) of the reference water are fixed, the enthalpy hi in kj/kmol and entropy s, in kJ/(kmol-K) are computed applying the IAPWS-IF97 formulation for the gas temperature and partial H2O pressure. 

Biomass differs from conventional fossil fuels in the variability of fuel characteristics, higher moisture contents, and low nitrogen and sulfur contents of biomass fuels. The moisture content of biomass has a large influence on the combustion process and on the resulting efficiencies due to the lower combustion temperatures. It has been estimated that the adiabatic flame temperature of green wood is approximately 1000°C, while it is 1350°C for dry wood [41]. The chemical exergies for wood depend heavily on the type of wood used, but certain estimates can be obtained in the literature [42]. The thermodynamic efficiency of wood combustors can then be computed using the methods described in Chapter 9. 

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