High Bridge Combined Cycle Plant

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1 High Bridge Combined Cycle Plant Location: Down town St. Paul, on the Mississippi River Plant Description: High Bridge is a combined cycle generating facility. A combined cycle plant produces electricity from two sources of energy: 1) Natural gas is used as a fuel in a combustion turbine, similar to a jet engine; 2) Exhaust from the combustion turbine also is used to make steam in a heat recovery steam generator. Both sources of energy drive turbine and electric generators to produce electricity. Integrating combustion and steam turbine technology provides an extremely efficient electricity production process. Power Production Capability: 570 Megawatts Fuel Source: Natural Gas Plant History: Built in 1923 as a coal-powered operation, the High Bridge plant, along with Riverside in Minneapolis, once formed the hub of Northern States Power Company, a predecessor to Xcel Energy. The original plant was replaced with a new natural gas fired generating facility starting in 2005 as part of Xcel Energy s Metro Emission Reduction Project. The coal-fired plant was retired in 2007 and the new facility came online in May Interesting Features: High Bridge gets its name from the bridge that crosses the Mississippi River just yards from the old plant. The plant site once was home to early residents of St. Paul; Many of them immigrants, who settled in shantytowns along the river. Environmental Highlights: The conversion of the High Bridge from coal to natural gas significantly reduced all air emissions from the plant: particulates by about 92%; nitrogen oxide by almost 97%; sulfur dioxide by 99%; and 100& of mercury emissions. High Bridge also has been a major sponsor and participant a Mississippi riverfront greening project in downtown St. Paul that involved the planting of hundreds of trees and shrubs. And it had been a participant with several other Xcel Energy plants in a successful peregrine falcon restoration effort. The falcon nest box that once was on the old plant stack has been successfully relocated to Archer-Midland-Daniels facility just a mile down Shepard Road. Community involvement: Located in the heart of downtown St. Paul, High Bridge and it employees are very active in the community. Employees deliver Meals on Wheels, volunteer at local schools, participating in the High School Intern Program with Johnson High School in St. Paul.t, and support the United Way and other community organizations. High Bridge employs 46 people, and property taxes on the plant benefit the city, county, and school system. Contact Information: Plant Information and Tour Request: Media Inquiries:

2 1. Natural Gas is piped in compress ed and burned in 2. A turbine engine similar to a jet engine. The heat from the engine is routed to 3. a boiler to turn water into steam which turned the blades of a turbine. 4. Each turbine spin generators to create electricity 5. A transformer increases the voltage and sends electricity over 6. distribution lines. Then local transformers reduce the voltage 7. for you to use.

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5 Major Equipment Combined Cycle Plant Combustion Turbine Generators (CTGs) The HBCC facility is based on the use of two Mitsubishi F class combustion turbines equipped with dry low Nox combustors. Each combustion turbine consist of the following equipment in series: an inlet air filter; a compressor, where air is drawn in and compressed; a combustor, where the air/fuel mixture is ignited; and a power turbine, where the combusted gases expand to rotate a turbine, cooled (as appropriate to enhance efficiency), compressed by the rotating compressor blades, and delivered to the combustor at substantially increased pressure and temperature. In the combustor, fuel (natural gas) is mixed with the inlet air and burned. The high temperature, high-pressure gas mixture then expands and moves across the angled turbine rotor blades, which are integrally connected to the combustion turbine rotor. The momentum of the expanding gases moving through the combustion turbine causes the compressor and turbine rotor blades and rotor to rotate. About 2/3 of energy procuded actually goes into the operation of the compressor section. The combustion turbine rotor shaft is directly connected to and drives an electrical generator that produces electrical power. The generator is driven at the compressor end to allow for axial exhaust to the HRSG. Modulating inlet guide vanes are used to improve the compressor low speed surge characteristics and for improved part load performance in combined cycle. The starting package includes an AC motor and hydraulic torque converter. The CTG rotor and turbine blades and nozzles are cooled with compressed air that is bled from the CTG compressor. The CTGs are housed in enclosures that provide thermal insulation, acoustical attenuation, and fire extinguishing media containment. A water wash skid to service the two CTGs was also be provided. Each CTG unit is approximately 150 feet long and of varying width and height. The widest and tallest component on the CTGs will be the air inlet filters, approximately 45 feet wide and 45 feet tall; These are located outside on the east side of the roof. Heat Recovery Steam Generators (HRSGs) The HBCC facility includes two HRSGs. The exhaust gases exit the combustion turbine and flow directly into the HRSG. Inside the HRSG, the hot exhaust gases are directed across the 14 different heat transfer tube surfaces causing the water in the tubes to boil and change phase into steam. The HRSGs are triple-pressure, reheat, natural circulation units, with supplemental natural gas firing to further improve steam-generating capability. The high, intermediate, and low-pressure sections of each HRSG consist of economizer, evaporator,

6 Major Equipment Combined Cycle Plant and superheater sections. The HRSGs are also equipped with natural gas fired duct burners that can be used to input additional heat to increase the steam generating capability of the HRSGs. As determined by permit requirements, the HRSGs include appropriate air pollution control equipment. Such as the selector Catalytic Reduction (SCR) sections. Each HRSG is approximately 105 feet tall, 60 feet wide (outside drums), and 125 feet long. Steam Turbine Generator (STG) One steam turbine is utilized for potential generation. The steam turbine receives steam produced by the two HRSGs. The steam turbine consists of three sections to extract the maximum energy from the steam. The STG is a reheat design, which means that exhaust steam from the high-pressure section will be returned to the HRSGs and reheated to increase steam cycle efficiency. High-pressure steam (main steam) received directly from the HRSGs will be expanded through the high-pressure turbine and will rotate the turbine shaft. The exhaust steam form the high-pressure turbine, or cold reheat steam, will be returned to the HRSGs, where it will be combined with steam from an intermediate-pressure section of each HRSG. The steam mixture will be reheated within the HRSG until its temperature approaches that of the main steam. This reheated steam, or hot reheat, will be directed to the intermediate pressure steam turbine. After the steam expands through the intermediate pressure turbine and imparts energy to the rotating turbine shaft, the steam combines with low-pressure turbine steam. The energy produced by the expanding steam in all threeturbine sections will rotate the turbine shaft, which is directly connected to and drives a generator to produce electrical power. Condenser Exhaust steam exiting the low-pressure section of the steam turbine will be condensed within a water-cooled steam surface condenser. The condensed steam collects in the bottom of the condenser, also called the hot well. Condensate pumps will pump this condensate from the condenser hot well back to the HRSGs to be reused to generate steam. Exhaust Stacks After passing through the HRSG, exhaust gases from each CTG will discharge through a steel stack. Each stack is approximately 19 feet in (inside) diameter. Heat Rejection System Cycle heat removed from the condensing steam in the condenser is absorbed by circulating water flowing through the condenser tubes. Heat absorbed by the circulating water is rejected to the river via the existing circulating water supply intake and discharge structures on the river. Gas Conditioning Station The gas condition station is located outdoors and supplies natural gas to a header supplying both CTGs and HRSG duct burners. The gas conditioning station receives natural gas from the header that supplies natural gas to the plant site.

7 Major Equipment Combined Cycle Plant CTGs, Duct Burners, Aux. Boiler, and HTG System The gas conditioning station includes fuel gas meters and pressure regulators for control and measurement of the gas being supplied. The gas conditioning equipment, is included to remove moisture and particulates from the gas stream supplying each CTG and the HRSG duct Burners.