Integrated Scheme of Natural Gas Usage with Minimum Production of Entropy

Abstract

A ccording to the second law of thermodynamics, any real industrial process is accompanied by an increase in entropy. An approach to reversible process conditions leads to a lower entropy increase in industrial processes and to more complete employment of feedstock and fuel resources (Ahern, 1980). However, this approach causes a decrease in process rates and an increase in equipment size. These two opposing tendencies should therefore be taken into account when seeking optimal process conditions.The various chemical reactions in industrial units are characterized by substantially different entropy production. In order to attain meaningful energy savings by approaching reversible conditions without noticeable increases in plant dimensions, it is the processes with extremely high entropy production that should be analyzed. One of these is the burning of organic fuels in power generation plants.Gas Turbine Units (GTUs), where the chemical exergy of natural gas (methane) is converted to electricity through consecutive stages of compression, combustion and expansion, are widely used. The goal of this article is to show the principal method to recovering that part of the energy that is wasted in the combustion chambers of the GTU by virtue of the irreversibility of fuel-burning by means of integration with the process of methane conversion into synthesis gas. The thermodynamic efficiencies of the standard GTU scheme and the integrated scheme based on our previous works (Safonov et al., 1993, Granovskii andSafonov, 1995) will be compared. To simplify the comparison of the different GTU cycles, the processes of compression and expansion are considered ideal and adiabatic, heat transfer in the heat exchangers is considered close to reversibility (almost perfect), and the temperatures of the mixing flows are equal. Also, the parameters of gaseous mixtures are calculated using the ideal gas station model. Standard Gas Turbine Cycle.Let us first consider the standard gas-turbine cycle, where electrical energy is derived from methane (natural gas). It has several stages: air compression in a compressor (5), methane combustion with air in a combustion chamber (1), gas expansion in a turbine (2) and heat transfer (3, 4) (Figure 1). The electrical energy, A, generated in the cycle, is: