How Does A Combined-Cycle Power Plant Work?

A Tour of the Metcalf Energy Center

Power Generation: 

Air Inlet

§  The amount of air needed for combustion is 800,000 cubic feet per minute.  This air is drawn though the large air inlet section where it is cleaned, cooled and controlled, in order to reduce noise.

Two Siemens Westinghouse 501FD Turbine-Generators:

§  The air then enters the gas turbine where it is compressed, mixed with natural gas and ignited, which causes it to expand.  The pressure created from the expansion spins the turbine blades, which are attached to a shaft and a generator, creating electricity.

§  Each gas turbine produces 185 megawatts (MW) of electricity.

§  The blades are attached to a rotor, which spins the generator, and makes electricity. Think of a generator as a huge spinning magnet inside a coil of wire. As the magnet spins, electricity is created in the wire loops.

Heat Recovery Steam Generator (HRSG)

§  The hot exhaust gas exits the turbine at about 1100 degrees Fahrenheit and then passes through the Nooter Erickson, Heat Recovery Steam Generator (HRSG).

§  In the HRSG, there are 18 layers of 100-foot tall tube bundles, filled with high purity water. The hot exhaust gas coming from the turbines passes through these tube bundles, which act like a radiator, boiling the water inside the tubes, and turning that water into steam.  The gas then exits the power plant through the exhaust stack at a much cooler 180 degrees, after having given up most of its heat to the steam process.

§  About 1 million pounds of steam per hour is generated in this way and sent over to the steam turbine through overhead piping. 

Steam Turbine

§  The steam turbine is a Siemens Westinghouse KN Turbine Generator, capable of producing up to 240 MW.  It is located on top of the condenser, across from the cooling tower. 

§  Steam enters the turbine with temperatures as high as 1000 degrees Fahrenheit and pressure as strong as 2,200 pounds per square inch. The pressure of the steam is used to spin turbine blades that are attached to a rotor and a generator, producing additional electricity, about 100 megawatts per HRSG unit. 

§  After the steam is spent in the turbine process, the residual steam leaves the turbine at low pressure and low heat, about 100 degrees.  This exhaust steam passes into a condenser, to be turned back into water. 

§  By using this “combined-cycle” process, two gas turbines and one steam turbine, we can produce a total of about 600 megawatts of electricity.

Emissions Control

Selective Catalytic Reduction (SCR)

§  To control the emissions in the exhaust gas so that it remains within permitted levels as it enters the atmosphere, the exhaust gas passes though two catalysts located in the HRSG.

§  One catalyst controls Carbon Monoxide (CO) emissions and the other catalyst controls Oxides of Nitrogen, (NOx) emissions. 

Aqueous Ammonia

§  In addition to the SCR, Aqueous Ammonia (a mixture of 22% ammonia and 78% water) is injected into system to even further reduce levels of NOx. 

Best Available Control Technology (BACT)

§  Our annual average concentration of NOx is only 2 parts per million, which is considered the “best available control technology” or BACT by the Air Board. 

§  As exhaust gas passes out of the exhaust stack, it is continuously sampled and analyzed, assuring that permit limits are being met. 

§  With this kind of clean, modern technology, the exhaust stack is only 145 feet high, compared to 500 feet, the height required by older power plants that use less efficient emission technology. 

§  Environmental and health organizations recognize this technology as a benefit to the community.  The local chapters of the American Lung Association and Sierra Club both support the Metcalf Energy Center. 

Transmission of Generated Power Onto the Grid

Transformers

§  The Gas Turbine and Steam Turbine generators produce power at 13,000 volts.

§  The transformers take the generated 13,000 volts and “transform” them to 230,000 volts, which is the required voltage needed for transmission to the nearby tower that sends power to the substation.

§  A small amount of generation is directed to “Auxiliary transformers” which “transform” the generated voltage to a lower voltage, so it may be used by the plant to power our own pumps, fans, and motors.  The Metcalf Energy Center requires 12 – 15 megawatts to operate.

Switchyard

§  From each transformer, the power passes underground into our switchyard.  The power from all of the generators comes together there, where it is measured, metered and directed onto the grid.

§  The proximity of the site to a large, existing PG&E substation makes it a good place to build a power plant and the nearest transmission tower is only about 200 feet away.

Condenser and Cooling Tower

§  The purpose of the condenser is to turn low energy steam back into pure water for use in the Heat Recovery Steam Generator.

§  The purpose of the cooling tower is to cool the circulating water that passes through the condenser. It consists of ten cells with large fans on top, inside the cone-like stacks, and a basin of water underneath.

§  We process and treat the Title 22 recycled water after receiving it from the City, before using it in our cooling tower.  The cool basin water absorbs all of the heat from the residual steam after being exhausted from the steam turbine and it is then piped back to the top of the cooling tower.

§  As the cool water drops into the basin, hot wet air goes out of the stacks. Normally, hot moist air mixes with cooler dry air, and typically a water vapor plume can be formed, one that may travel hundreds of feet in the air and be seen from miles away.  The California Energy Commission considered this visually undesirable in this community so we added a “Plume-Abatement” feature, louvers along the topsides of the tower that control the air flow.  

§  The cooling tower evaporates about three-fourth’s of the processed, recycled water, then we send about one-fourth of it back through the sewer lines for re-treatment by the City. 

§  The Metcalf Energy Center purchases 3 to 4 million gallons per day of recycled water from the City of San Jose. Evaporation of this water assists the City in adhering to their flow cap limits and helps to protect the sensitive saltwater marsh habitat of the San Francisco Bay environment from receiving too much fresh, recycled water.

Water Tanks, Natural Gas Pipeline, Control Room

Water Tanks

§  The largest tank is the Service Water tank.  It contains 470,000 gallons of water to be used for drinking, fire fighting and for the high purity water train. The water from the service water tank is pumped to the water treatment building where it then passes through a reverse osmosis unit, a membrane decarbonater, and mixed resin bed demineralizers to produce up to 400 gallons per minute of ultra pure water.

§  The pure water is then stored in the smaller 365,000-gallon tank until it is turned into steam for making electricity.

Natural Gas

§  Natural gas fuels the combustion turbines. Each turbine can consume up to 2,000 MMBTU per hour.

§  The fuel comes from the major high pressure natural gas pipeline that runs along the east side of Highway 101, less than 1 mile to the east of our site. 

§   During construction, “Horizontal Directional Drilling” was utilized with careful coordination with many local authorities.  The pipeline was built 60 feet underground and passed under highways, creek, train tracks, and environmentally sensitive areas. 

§  The pipeline enters the site just behind the water tanks, where equipment regulates and measures the natural gas composition, flow and pressure. 

§  Gas compressors pump the natural gas though the facilities’ fuel gas system where it is delivered to the gas turbine and the HRSG duct burners at the proper temperature, pressure and purity.

Control Room

§  From the control room, the plant operators monitor and operate the facility, via the plant’s “Distributed Control System”, with the click of a mouse, viewing graphic representations of all MEC systems on various screens.  

§  The system gives operators both audible and visual signals to keep them informed of plant conditions at all times and to determine when preventative maintenance is required.