Environmental Engineering Reference
In-Depth Information
the problems. A 30% decrease over 2 or more hours may not have the impact
of an instantaneous 10% decrease.
The emissions instability associated with cycling is a function of the ages
and designs of individual plants and reflects the inherent operational diffi-
culties associated with coal-fired facilities. If a coal-fired plant must cut back
its output, the input rate of the feed coal must be cut to produce a lower rate
of steam generation at the correct temperature to maintain low NO
X
genera-
tion. This is not as simple as it sounds. A boiler is designed to run at certain
heat output. At lower output, the boiler may be too large to maintain the
output at the desired temperature.
Think of the automobile example again. Imagine a car engine specifically
designed to run on flat highways (like a utility boiler). The engine and cool-
ing system were designed to operate at an optimal temperature to achieve
the lowest energy consumption and emissions level for the amount of power
produced. If you were to drive this car downhill, the engine would generate
too much power for the conditions and the engine must be throttled back.
With lower power output, the engine would tend to run at a lower temperature
because the cooling system was designed to take away far greater amounts of
heat than are being generated. Likewise, when the car must run uphill and
requires more power, the cooling system may not be capable of evenly cooling
the engine. The uneven temperatures within the engine will lead to suboptimal
operating conditions. Hot spots in the engine may cause premature ignition,
resulting in lower mileage and higher emissions. The engine will require more
fuel to generate the same amount of power while emissions will increase.
Varying the operation conditions of a complex combustion system in which
a precise and steady flame temperature coupled with precise amounts of fuel
and air to maintain efficient and clean combustion poses a great challenge.
Boilers are such systems and are designed to run most efficiently within a
narrow, steady-state range of operating rates.
The combination of operating efficiently and controlling emissions
requires a complex mix of computer-based technology and manual inter-
vention. As many as 50 adjustments may be required to maintain fuel-to-air
mixes and lime-slurry mixtures for proper SO
2
absorption in response to
changing generation output. Although computerized controls are employed,
determining exact adjustments is not always a straightforward process.
4
With changing conditions, the combustion processes are frequently subopti-
mal and the calculated adjustments may not produce the expected impact on
boiler operation. These irregularities cause unstable operation and require
manual adjustments—and when manual adjustments must be made, a plant
is subject to the greatest risk of instability. Significant emission excesses may
result from a suboptimal flame, leading to lower efficiency, partial loss of
flame, and in an extreme case, a total plant shutdown.
Another serious consequence of cycling coal plants is plant damage. The
financial cost of correcting the damage would include an immediate increase
in plant maintenance expense and a reduction of useful plant life—very
