Steam
Rankine cycles can be combined with topping and/or bottoming cycles to form
binary thermodynamic cycles. These topping and bottoming cycles use working fluids
other than water. Topping cycles change the basic steam Rankine cycle into a
binary cycle that better resembles the Carnot cycle and improves efficiency.
For conventional steam cycles, state-of-the-art materials allow peak working fluid
temperatures higher than the supercritical temperature for water. Much of the
energy delivered into the cycle goes into superheating the steam, which is not
a constant-temperature process. Therefore, a significant portion of the heat
supply to the steam cycle occurs substantially below the peak cycle temperature.
Adding a cycle that uses a working fluid with a boiling point higher than water
allows more of the heat supply to the thermodynamic cycle to be near the peak
cycle temperature, thus improving efficiency. Heat rejected from the topping
cycle is channeled into the lower-temperature steam cycle.
Thermal
energy not converted to work by the binary cycle is rejected to the
ambient-temperature reservoir.
Metallic
substances are the working fluids for topping cycles. For example, mercury was
used as the topping cycle fluid in the 40-MW plant at Schiller, New Hampshire.
This operated for a period of time but has since been dismantled. Significant
research and testing has also been performed over the years toward the eventual
goal of using other substances, such as potassium or cesium, as a topping cycle
fluid.
Steam power
plants in a cold, dry environment cannot take full advantage of the low heat
rejection temperature available. The very low pressure to which the steam would
be expanded to take advantage of the low heat sink temperature would increase
the size of the low-pressure (LP) turbine to such an extent that it is
impractical or at least inefficient. A bottoming cycle that uses a working fluid
with a vapor pressure higher than water at ambient temperatures (such as
ammonia or an organic fluid) would enable smaller LP turbines to function efficiently.
Hence, a steam cycle combined with a bottoming cycle may yield better
performance and be more cost-effective than a stand-alone Rankine steam cycle.
Steam Boilers
A
boiler, also referred to as a steam generator, is a major component in the
plant cycle. It is a closed vessel that efficiently uses heat produced from the
combustion of fuel to convert water to steam. Efficiency is the most important
characteristic of a boiler since it has a direct bearing on electricity
production.
Boilers
are classified as either drum-type or once-through. Major components of boilers
include an economizer, superheaters, reheaters, and spray attemperators.
Drum-Type Boilers
Drum-type
boilers (Figure 6) depend on constant recirculation of water through some of the
components of the steam/water circuit to generate steam and keep the components
from overheating. Drum type boilers circulate water by either natural or
controlled circulation.
Natural Circulation.
Natural
circulation boilers use the density differential between water in the down
comers and steam in the water wall tubes for circulation. Controlled Circulation.
Controlled
circulation boilers utilize boiler-water-circulating pumps to circulate water
through the steam/water circuit.
Once-Through Boilers
Once-through
boilers, shown in Figure 7, convert water to steam in one pass through the system.
Major Boiler Components
Economizer.
The
economizer is the section of the boiler tubes where feedwater is first
introduced into the boiler and where flue gas is used to raise the temperature
of the water.
Steam
Drum (Drum Units Only).
The steam
drum separates steam from the steam/water mixture and keeps the separated steam
dry.
Superheaters.
Superheaters
are bundles of boiler tubing located in the flow path of the hot gases that are
created by the combustion of fuel in the boiler furnace. Heat is transferred
from the combustion gases to the steam in the superheater tubes.
Superheaters
are classified as primary and secondary. Steam passes first through the primary
superheater
(located in
a relatively cool section of the boiler) after leaving the steam drum. There
the steam receives a fraction of its final superheat and then passes through
the secondary superheater for the remainder.
