Chemical Engineering April 2013 - 49

Source: ATCO Power
Electricty
sales
to grid
Transformer
Gas
Natural gas
turbine
Electricity
sales to host
Electricty
Station
services
Steam
turbine
Generator
High pressure
steam sales
to host
Low-pressure
steam sales
to host
Gas
turbine
Heat recovery
steam
generators
Energy
host
Figure 2.
A typical cogeneration
configuration is shown here
one common cogeneration configuration
(Figure 2), natural gas is burned
in a combustion turbine connected to
an electric generator to produce electricity.
The hot exhaust gases from
the combustion turbine are routed
into a heat recovery steam generator
(HRSG), a steam boiler that uses
the heat in the exhaust gases (sometimes
supplemented with additional
natural gas firing) to heat feedwater
into steam. That steam can be used
to power a steam turbine connected
to another load (an electric generator
that makes electricity for the plant, or
to drive a large piece of equipment),
or to supply steam directly for process
heating needs, or both.
Another common example of cogeneration
is the use of a traditional
" topping boiler " to produce high-pressure
steam that generates electricity
in a steam turbine-generator, which
then exhausts steam at lower temperatures
and pressures to match the
thermal needs of the manufacturing
processes. In general, cogeneration
processes can be configured in a variety
of ways to accommodate the ratio
of thermal-to-electric energy required,
temperatures and pressures of steam
needed by manufacturing or mechanical
drive systems, the scale of the system,
the types of fuels available, and
a variety of site-specific technical and
economic factors.
Adapting and installing cogen
While most CPI plants can be configured
to incorporate cogeneration
processes, there are a few common
hurdles that engineers must recognize
and overcome when implementing the
necessary technology.
Energy balance. The primary concern
for engineers is to balance the
ratio of thermal energy to electrical energy.
This ratio can shift significantly
depending on the facility's load profile
and its appetite for exporting electricity
to the grid, and is one of the key
variables that will dictate the design
engineer's choice of the prime mover.
In general, a project with a high ratio
of thermal-to-electric power (for example,
80% thermal to 20% electrical)
will generally favor a topping boiler.
Conversely, a project with a low ratio
of thermal-to-electric power (for instance,
60% thermal to 40% electrical)
will generally favor a combustion turbine
with a HRSG - with or without
a steam turbine generator.
The largest integrated cogeneration
site of the author's company in
Kingsport, Tenn. has operated independently
from the electricity grid for
decades by carefully matching the significant
thermal demand growth with
boilers and both turbine generators
and mechanical-drive turbines. The
blend of electric motors and mechanical-drive
turbines resulted in a robust
system with a variety of flowpaths to
move steam between different pressure
levels while ensuring both reliability
and thermal efficiency. The
company's integrated cogeneration
site in Longview, Tex. uses a combustion
turbine cogeneration system and
takes advantage of the opportunity to
export electricity to the grid while also
supplying steam to manufacturing assets
at the site.
Scale. Another key consideration
for engineers implementing cogeneration
processes is the scale of the
project. The large capital cost of installing
combustion turbines or topping
boilers can be justified if large,
stable energy demands exist (for
example, tens or hundreds of megawatts
of electric demand). However, a
different kind of prime mover would
be more suited to small energy demands.
For example, a reciprocating
internal combustion engine with
heat recovery from the jacket cooling
water may be appropriate for a sysChemiCal
engineering www.Che.Com april 2013 47
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Chemical Engineering April 2013

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