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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.
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Citation
Sama, D.; Sanhong, Q. (1988). Second Law Analysis for Process and Energy Engineering: Use in a Steam/Power Cycle. Energy Systems Laboratory (http://esl.eslwin.tamu.edu). Available electronically from https : / /hdl .handle .net /1969 .1 /92451.