High density combined cycle power plant process

Details High density combined cycle power plant process High density combined cycle power plant process

2001-02-14

2002-12-17

Casaregola, Louis J. (Department: 3746)

Power plants

Combustion products used as motive fluid

Process

C060S039281

Reexamination Certificate

active

06494045

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to combined cycle power plants that may or may not incorporate cogeneration into their cycle. As will be demonstrated by the following disclosure, the increasing need for more energy efficient and environmentally friendly methods of generating power has prompted a widespread search for systems and methods to achieve these goals. However, current technologies have a generally myopic view of the total economic impact imposed by a concentration on energy efficiency and environmental issues alone.
The present invention proposes to break with tradition and include as part of the economic and environmental analysis the complete equipment complement required to implement a desired plant load (power) rating. By incorporating this analysis into a new system and method of supplemental firing and heat recovery, the present invention dramatically cuts the overall economic and environmental cost of installed power plants by reducing the equipment complement while maintaining or reducing plant emissions. The result of this improvement over the art is cheaper and cleaner electrical energy than would be possible using conventional combined cycle plants that are currently known in the art.
BACKGROUND OF THE INVENTION
Overview
Combined cycle power plants and cogeneration facilities utilize gas turbines (GT(s)) as prime movers to generate power. These GT engines operate on the Brayton Cycle thermodynamic principle and typically have high exhaust flows and relatively high exhaust temperatures. These exhaust gases, when directed into a heat recovery boiler (typically referred to as a heat recovery steam generator (HRSG)), produce steam that can be used to generate more power and/or provide process steam requirements. For additional power production the steam can be directed to a steam turbine (ST) that utilizes the steam to produce additional power. In this manner, the GT produces work via the Brayton Cycle, and the ST produces power via the Rankine Cycle. Thus, the name “combined cycle” is derived. In this arrangement, the GT Brayton Cycle is also referred to as the “topping cycle” and the ST Rankine Cycle is referred to as the “bottoming cycle,” as the topping cycle produces the energy needed for the bottoming cycle to operate. Thus, the functionality of these cycles is linked in the prior art.
Rankine Cycle
Steam has been used for power applications for more than a century. Early applications utilized a pump to bring the water up to the desired pressure, a boiler to heat the water until it turned to steam, and a steam engine, typically a piston type engine, to produce shaft horsepower. These power plants were used in factories, on locomotives, onboard steamships, and other power applications.
As technology progressed, the trend for the use of steam engines diminished and the use of steam turbines increased. One advantage of the steam turbine was its overall cycle efficiency when used in conjunction with a condenser. This allowed the steam to expand significantly beyond normal atmospheric pressure down to pressures that were only slightly above an absolute vacuum (0.5 to 2 pounds per square inch absolute (psia)). This allowed the steam to expand further than in an atmospheric exhaust configuration, extracting more energy from a given mass of steam, thus producing more power and increasing overall steam cycle efficiency. This overall steam cycle, from a thermodynamic perspective, is referred to as the Rankine Cycle.
FIG. 1
illustrates the thermodynamic operation of the Rankine Cycle. In
FIG. 1
, graph (
100
) illustrates the Rankine Cycle on a Pressure versus Volume plot. From point (
101
) to point (
102
), water is pressurized at constant volume. From point (
102
) to point (
103
), the water is boiled into steam at constant pressure. Point (
103
) to point (
104
) defines the process where the steam expands isentropically and produces work. Then, from point (
104
) to point (
101
) the low-pressure steam is condensed back to water and the cycle is complete.
Also in
FIG. 1
, graph (
110
) illustrates the Rankine Cycle on a Temperature versus Entropy plot. From point (
111
) to point (
112
), water is pressurized. From point (
112
), the water is boiled into steam at constant temperature until it is all steam, then it is superheated to point (
113
). Point (
113
) to point (
114
) defines the process where the steam expands isentropically and produces work. From point (
114
) to point (
111
) the low-pressure steam is condensed back to water at constant temperature to complete the cycle. See Eugene A. Avallone and Theodore Baumeister III, MARKS' STANDARD HANDBOOK FOR MECHANICAL ENGINEERS (NINTH EDITION) (ISBN 0-07-004127-X, 1987) in Section 4-20 for more discussion on the Rankine Cycle.
Power Plant Cycle
For a number of decades, the Rankine Cycle has been used to produce most of the electricity in the United States, as well as in a number of other countries.
FIG. 2
illustrates a schematic of the basic Rankine Cycle, with the four primary components being the Boiler Feed Pump (BFP) (
201
), Boiler evaporator/superheater (BOIL) (
203
,
205
), Steam Turbine (ST) (
207
), and the Condenser (COND) (
209
). Note that either one or multiples of any component are possible in the arrangement, but for simplicity, only one of each is shown in FIG.
2
. The sub-critical Rankine Cycle (steam pressures less than 3206.2 psia) starts as water at the inlet (
211
) of the BFP (
201
). The water is then pumped to a desired discharge pressure by the BFP (
201
). This pressurized water (
202
) is then sent to the evaporator (EVAP) (
203
) where heat is added to the pressurized water. Typically this is accomplished by burning a fuel in the boiler, and the heat of combustion is then transferred to the pressurized water that is routed through tubes and other passages and/or vessels in the boiler. As sufficient heat is added to the pressurized water, it boils and turns into steam (
204
). This steam now exists in the two-phase region where both steam and water coexist at the same pressure and temperature, called the saturation pressure and saturation temperature. For most applications designed in recent decades, this steam (
204
) is then sent to a superheater section (SHT) (
205
) in the boiler where it is heated to a higher temperature than saturation temperature. This steam (
206
) is now referred to as superheated steam. Superheated steam reduces (but does not eliminate) the risk of water carryover into the steam turbine (
207
), which is of concern since water carryover can cause extensive internal steam turbine damage. Of more importance, however, is the fact that superheated steam yields better cycle efficiencies. This is of great importance to large central power stations.
Once produced, the superheated steam (
206
) is sent to the steam turbine (
207
), typically via one or more pipes. The steam then begins to expand in the steam turbine (ST) and produce shaft horsepower. After traveling through the steam turbine down to a low exhaust pressure, the steam exits the ST (
208
), and is sent to the condenser (
209
), where it is then condensed back into water. This device is typically a tubed heat exchanger, but can also be other types of heat exchangers such as a spray chamber, air-cooled condenser, or other heat exchange device used for a similar purpose. After rejecting heat from the low-pressure steam and condensing the steam back to water, the condenser collects the water in an area commonly referred to as the hotwell (HW) (
210
), where it is then typically pumped through the condensate line (
211
) and back to the BFP (
201
). Shaft horsepower produced in the ST is converted into electrical power in the generator (GEN) (
212
). This cycle of one unit of water from the point of beginning, through the system, and back to the point of origin defines the basic Rankine Cycle.
Current power plants using only steam as the motive fluid typically use a boiler to produce the steam. This boiler may be fueled by a variety of fuels, including oil, natura

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