Chemical exergy methane

This work is called the chemical exergy of methane and is the maximum amount of work that this compound has available for performing work in the environment (Figure 6.3). Indeed, natural gas is an important fuel for a power station. Table 6.1 gives the chemical exergy or available work of a number of compounds energy carriers, raw materials, and pure products. 

Table 6.2 lists exergy values for methane. It is clear from this table that methane carries an impressive amount of exergy as chemical exergy. Further, the table shows (1) the influence of increased pressure and temperature on the physical exergy and (2) that this latter contribution of exergy is nearly two orders smaller than the chemical contribution. Chemical exergy is the exclusive subject of Chapter 7. 

Table 7.3 is useful for the calculation of the standard chemical exergy values of compounds. We illustrate this for methane and start from its hypothetical formation reaction at standard conditions ... 

The conditions of a natural gas reservoir are 30 MPa and 100°C. The gas, assumed to be pure methane, is spontaneously expanded to a pressure of 7MPa (Figure 8.1). Assuming that this expansion is adiabatic, calculate the amount of work that is lost in the process, and express it as a fraction of the originally available amount of work per mole of gas in the reservoir. Carry out this calculation while making a distinction between the physical and chemical exergy of the gas. 

Next, we wish to calculate which fraction this lost work is of the work originally available in the gas. The chemical exergy of the gas, assumed to be methane, is significant, 831.65 kj/mol, but it should be excluded from the calculation because no chemistry is involved in the expansion step. The work available in the gas at initial and final conditions can be calculated from Equation 6.11 ... 

An efficient plant must be based on making an approach, as near as is practical and economic, to the perfect processes of an equilibrium diagram, such as Figure A.4, which was initially composed as a calculation route for methane chemical exergy, and then realised to have larger implications. Methane had to be consumed in an isothermal equilibrium reversible process. 

The process of the calculation involves a reformer which gets its exergy not from combustion, but as electrical power generated by supplying CO and Fi2 to separate fuel cells which are able to create an excess above the need of the reformer. The invaluable JANAF thermochemical tables (Chase etah, 1985) provided the thermodynamic data. The excess is the chemical exergy of methane. 

There are parallel achievements at the University of Pennsylvania, (Park etai, 1999 2000 2001 Gorte etai, 2000), using anodes with copper substituted for nickel to avoid carbon formation. The last two papers include the electrochemical oxidation of dry fuels other than methane, for example gasoline and diesel, the chemical exergy of which is difficult to calculate, since they are mixtures requiring separative work. 

CHEMICAL EXERGY OE METHANE AND RELATED HIGH-EFHCIENCY HYDROGEN PRODUCTION... 

A.3.10 Route 2 Fuel Chemical Exergy by Direct Methane Oxidation... 

When this appendix was in preparation, methane electrochemical oxidation had not been achieved. A reformer was essential That fact influenced the author s choice of initial calculation route for the methane chemical exergy, to be via oxidation via an equilibrium reformer. Meanwhile direct oxidation has been achieved in the laboratory, as mentioned in Section A.3.2 (route 1). 

The Oxidation is Isothermal at To, leading to the calculation of the Methane Chemical Exergy... 

Figures A.4 and A.5 were drawn initially in the pursuit of a calculation route for the chemical exergy of methane. For that purpose the two equilibrium diagrams each provide a reversible route to power production from methane. Each process gave a confidence-raising, similar answer for the chemical exergy of methane.

For the power plant of Example 5.8, assuming that the fuel is methane and using the heat of combustion and chemical exergy data of Example 5.9 ... 

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