Page 124 - Sustainability in the Process Industry Integration and Optimization

P r o c e s s I n t e g r a t i o n f o r I m p r ov i n g E n e r g y E f f i c i e n c y 101

4.6.4 Power Cogeneration
The factors that control the economics of utility systems are fuels and
their properties, the ratio of fuel prices to power prices, efficiency of
the utility system, and the amount of power to be imported/exported.
Depending on these factors and the Total Site power demand, the site
may be a net power importer or exporter or be in power balance.
Most industrial processes require steam at different pressure
levels up to about 30 bar. Central utility boilers usually generate steam
at higher pressure (40–100 bar). Back-pressure steam turbines are used
to expand the steam from higher- to lower-level steam headers, thus
generating power while delivering steam to processes. Another
method of cogeneration employs a gas turbine, which is in itself a
power generating equipment. A gas turbine produces large amounts
of waste heat along with the power—the ratio of heat to power is
about 1.5–2. This is high-temperature (450–600°C) waste heat capable
of generating even VHP steam. The heat from a gas turbine’s exhaust
stream can be utilized by heat recovery steam generators with or
without supplementary firing. The generated steam is expanded
through steam turbines to produce additional power.
The work of Dhole and Linnhoff (1993b) has been further
developed by Raissi (1994) and Klemeš et al. (1997). The latter paper
describes the development of a tool called the Site Utility GCC
(SUGCC). The area enclosed by this curve is proportional to the
power cogeneration potential of the site steam system; Klemeš et al.
(1997) also defined a simple proportionality coefficient, whose value
is usually evaluated for each industrial site separately. This
cogeneration targeting model is referred to as “the T-H model”
because it is based on heat flows through the steam system.
Using SUGCCs allowed Klemeš and colleagues (1997) to set
thermodynamic targets for cogeneration of power along with targets
for site-scope heat recovery that would minimize the cost of utilities.
Satisfying the goal of maximum heat recovery leads to a minimum
boiler VHP steam requirement, which in turn can be achieved by
maximizing steam recovery. Here the power generation by steam
turbines is also minimal, which has the effect of maximizing
imported power. This scenario can be represented by the Site CCs
that are shifted to a position of maximum overlap (i.e., pinched). This
target represents the thermodynamic limitation on system efficiency,
but this is not a specification that must be achieved. The case of
minimizing the cost of utilities is handled by exploring the trade-off
between steam recovery and power cogeneration by steam turbines.
If design guidelines are thus based on minimizing cost, then the
resulting network design is usually different from that produced
when aiming to minimize fuel consumption.
Mavromatis and Kokossis (1998) proposed a simple model of
back-pressure steam turbine performance. In this model, the
performance of a steam turbine is related to its size (in terms of