Second Law Analysis for Process and Energy Engineering: Use in a Steam/Power Cycle

Abstract

The objective of this paper is to demonstrate how the use of the 2nd Law can help an engineer make intelligent process design decisions in a relatively simple and straightforward manner. A sensitivity to 2nd Law inefficiencies, when used with common sense, can lead to alternative design improvements. Looking at a process from a 2nd Law viewpoint gives the engineer a better insight into the process. As a hypothetical example we will examine a conceptual design of an industrial utility plant that generates steam used to provide process steam and power needs. A flowsheet for this plant, with energy and mass flows as well as steam conditions, is shown in Figure 1. The average manufacturing plant power requirement is projected to be 10 Mw; the interconnect with a public utility provides power for manufacturing when the requirements exceed the capacity of the plant in Figure 1. Conversely, when the capacity exceeds the manufacturing needs, the excess can be sold to the utility. For the case study at hand it is to be assumed that the cost of electricity per kwh is four times that of fuel (based on higher heating value). The plant uses a coal-fired furnace to produce steam at 100 bars pressure and 500°C (1450 psia and 930°F), and provides steam to headers for process users at 80, 40 and 4 bars (1160, 580 and 58 psia). The steam is delivered by extraction from a back pressure turbine which has an isentropic efficiency of 75%. The boiler efficiency is 85% of the higher heating value (HHV). At first glance this seems like an efficient design for an industrial steam plant.