Stonewall’s ability to bring Loudoun County its own clean power source is possible because of the type of plant we plan to build. According to many environmental organizations, old fashioned power plants we currently use are on average 38 percent efficient. That means many of the power plants that fuel our lives today are throwing away two-thirds of the energy they create. That’s not smart. That’s not clean. That’s not green. The combined cycle generation technology included as part of the plant will have an efficiency of approximately 58%, far superior to that of older generating plants that now serve Loudoun County. It is expected to operate as a primary or intermediate load plant depending on daily and seasonal power demand.
This section will explain in detail the various components that will work together in the Stonewall Energy facility.
We also encourage you to read more about CHP systems online at Greenpeace’s Web site.
- Air Inlet:
The air is drawn through the large air inlet section where it is filtered, cooled, and controlled in order to reduce noise.
- Two Gas 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 about 215 megawatts per hour of 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 and sent out to the grid.
- Heat Recovery Steam Generator:
The hot exhaust air exits the gas turbine at about 1100 degrees Fahrenheit and then passes through the Heat Recovery Steam Generator (HRSG). In the HRSG, there are numerous layers of vertical tube bundles filled with high purity demineralized water. The hot exhaust air 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 air then exits the power plant through the exhaust at a much cooler 180 degrees, after having given up most of its heat to the steam process. This steam is used to drive a steam turbine generator, which in turn produces an additional 320 megawatts per hour of electricity, thus greatly increasing the overall energy efficiency of the process.
- Emissions – Best Available Control Technology (BACT):
Before any air passes out of the exhaust stacks of the power plant, it is constantly sampled, analyzed, and controlled, thus assuring that stringent air quality limits set and regulated by the State of Virginia are being met. To control the emissions in the exhaust gas so that it remains within permitted levels as it enters the atmosphere, several emissions control technologies and systems are utilized Oxides of Nitrogen (NOx) are controlled during the combustion process in the gas turbines, and further reduced using Selective Catalytic Reduction (SCR), where aqueous ammonia (a mixture of 81 percent water and 19 percent ammonia) is injected into the gas stream as it passes through a catalyst in the HRSG before it is sent out of the exhaust stack . Another, an oxidation catalyst, is used to control carbon monoxide (CO) emissions.
- Transmission of Generated Power onto Grid:
- Transformers: The Gas Turbine and Steam Turbine generators produce power at 13,000 volts. The transformers take the generated electricity at 13,000 volts and increase it to 230,000 volts, which is the required voltage needed for distribution to the transmission grid. A small amount of electricity is directed back to lower voltage transformers in the plant so it may be used by the plant to power pumps, fans, and motors.
- Switchyard: From each transformer, the power passes into a switchyard located on site. The power from all of the generators comes together there, where it is measured, metered and directed onto the grid. The proximity of this site to the Pleasant View and Loudoun substations makes it a good place to build a power plant.
- Condenser and Cooling Tower:
The condenser is required to turn steam back into water for re-use in the Heat Recovery Steam Generator. The cooling tower cools the circulating water that passes through the condenser. It consists of 10-12 cells with large fans on top inside cone-like stacks, and a basin of water underneath. The recycled effluent waste water is treated by the Town of Leesburg before using it in the cooling tower. The basin of cool 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. The project will use modern plume abatement technology to mitigate presence of a water vapor plume. Louvers along the topsides of the tower control the air flow. The cooling tower evaporates about 90% of the processed, clean, recycled water it takes in.
- Water Tanks, Natural Gas Pipeline, Control Room:
- Water Tanks: The largest tanks on site will be used to store water for the cooling tower that is lost through evaporation. Other tanks will store pure, demineralized water that is used in the production of steam, as well as the aqueous ammonia and other liquids used for various plant processes.
- Natural Gas: Natural gas fuels the combustion turbines. The gas comes from the major high-pressure natural gas pipelines that run through the property. The natural gas is drawn directly from the pipelines on-site and is directed to equipment that regulates and measures the natural gas composition, flow, and pressure before it is used in the power generation equipment.
- Control Room:From the control room, the plant operators continuously monitor and operate the facility around the clock. With the click of a mouse, graphic representations of all the plant systems appear on various screens. The system gives operators both audible and visual signals to keep them informed of plant conditions at all times.