ABSTRACT
Objectives ◾ Introduce the concept on availability analysis for reacting systems ◾ Apply availability concept to fuel cells and estimate the maximum possible voltage ◾ Conduct availability analyses for reacting systems ◾ Present physical meaning for maximum possible work for a combustion system ◾ Define fuel availability and present relation between fuel availability and heat value
13.1 Introduction Combustion is a process during which chemical energy is converted into thermal energy. Œe ultimate objective of a combustion process is to convert chemical energy of fuel into useful work. Œe extent to which this conversion is possible can be determined through an availability analysis of combustion process. We have discussed availability concepts for nonreacting systems in Chapter 4. Œis chapter presents availability analyses for reacting systems. Chemical reaction is the dominant process for exergy loss in combustion, and the exergy expressed as a % of heat value changes in relation to temperature. Œe exergy loss in the gas turbine combustor could be as high as 20-30% of heat value and forms the largest of all losses in a gas turbine system. On the contrary for combustion, thermal gradient instead of chemical reaction is the major process for exergy loss in nonreacting thermal systems. We will also discuss a methodology of determining the maximum possible work potential of various fuels and give physical meaning of maximum work for chemical systems. For instance, in this context, it is often required to determine the maximum possible work per gallon of gasoline or other liquid fuel produced from an automobile engine operating in the ambient. Finally the availability concepts are used in determining maximum possible voltage in a fuel cell. Further applications of availability theory to biological systems, human organs, and plants are presented in Chapter 14.
