Chemical exergy thermodynamic efficiency

Part II, "Thermodynamic Analysis of Processes" (Chapters 6 through 8), discusses the thermodynamic efficiency of a process and how efficiency can be established and interpreted. A very useful thermodynamic property, called exergy or available work, is identified that makes it relatively easy to perform and integrate the environment into such an analysis. Some simple examples are given to illustrate the concept and its application in the thermodynamic or exergy analysis of chemical and nonchemical processes. 

Suppose we deal with a process in which iron, Fe, has to be used as a reactant, for example, in a reduction reaction. The standard chemical exergy of Fe is 376.4 kj/mol. If we wish to carry out a thermodynamic or exergy analysis of this process, this value is not appropriate. After all, to put the exergy cost of the product, for which Fe was needed as a reactant, in proper perspective, we need to consider all the exergetic costs incurred in order to produce this product all the way from the original natural resources— iron ore and fossil fuel in this example. The production of iron from, for example, the iron ore hematite and coal has a thermodynamic efficiency of about 30% [1], and therefore it is not 376.4 kj/mol Fe that we need to consider... 

If the thermodynamic efficiency of a process step is calculated, the chemical exergies should be excluded from the calculation if the process step does not include chemical conversions. If it does, it may be appropriate to distinguish between the physical and the chemical efficiency, itphys and T chem, °f the process step. 

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. 

N2 and H2 and the production of H2 from natural gas. Assume physical steps to have efficiencies of 10% and chemical steps of 60%. What is the cumulative exergy consumption of urea Separations take place with the help of compressors with an efficiency of 75%, running on electricity from an advanced natural gas-fired power station with a thermodynamic efficiency of 55%. 

Shakespeare s fairyland is mirrored in equilibrium thermodynamics all is simplicity and perfection For fuel cells, the gist of such a theory, tackled by Gardiner (1996), but challenged by Appleby (1994), is that the irreversible losses inherent in practical systems must be separated and evaluated. Then a comparison of practical with perfect, via a summation of the losses, leads to a calculated and understandable efficiency. The latter is an underlay to the economics, the final arbiter. The notion that the calorific value of the fuel, as distinct from its much larger chemical exergy, is a basis for performance calculations has been dismissed by Barclay (2002). In the foreword of this book, it is predicted that the novel ideas herein will get over, but rather slowly. But the ideas are not challenged. 

The thermodynamic basis of the calculation of the maximum possible work potential or chemical exergy of reversible and irreversible chemical reactions is explained and discussed. Combustion is asserted to be fundamentally irreversible. It is a nonequilibrium uncontrollable chain reaction with hot branches, in a cool milieu, and a limited work output proportional to Carnot efficiency x calorific value (Barclay, 2002). 

Second-law thermodynamic analyses have been shown to be of considerable value when applied to systems where an efficient energy interconversion is important. Using the chemical energy transport systems as examples, the use of exergy ratios as a measure of thermodynamic quality has been shown to give Important Insights Into the efficiency or inefficiency inherent in any conversion of one energy form to another. 

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