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Résumé :
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Estimation of chemical exergy of biomass is one of the basic steps in performance analysis and optimization of biomass conversion systems. A practical method for estimating specific chemical exergy of biomass on dry basis (db) from basic analysis data was developed on Szargut's reference environment model. The method is based on exergy and entropy equations of reaction, Gibbs free energy relations, a modified estimation of the standard entropy of organic matter in biomass and an assumption about original state of inorganic matter-forming elements in biomass. The method was applied to 86 varieties of biomass, and the statistical results indicate that specific chemical exergy of dry biomass varies in the interval of 11.5―24.2 MJ·kg―1, which is always slightly larger than the higher heat value (HHV) (db). Owing to the relative very small value, the influence of inorganic matter in the form of chemical exergies of ash and oxygen reacting with inorganic matter, and the entropy change in ash formation can be neglected. The average ratio of specific chemical exergy to HHV is 1.047 for dry biomass. Consequently, specific chemical exergy of dry biomass can be conveniently estimated from ultimate analysis data plus ash content (in wt %, db) or HHV.
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