St. Paul’s new $428-million, 580-MW High Bridge powerplant will more than double the output of the city’s aging facility
by Tony Illia
The sheathed High Bridge plant will protect machinery from the elements,prolonging its life span in the frigid Minnesota winters.
Photo courtesy of Xcel Energy by Rick Souther
The High Bridge powerplant, an industrial landmark along the riverfront in St. Paul, Minn., is being replaced by a newer, more efficient model.
The four-unit, coal-fired electrical generating facility just outside of downtown gets its name from another local landmark called the High Bridge, which dates back to 1889. The five-span steel bridge crosses the upper Mississippi River a half-block southeast of the plant.
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The High Bridge plant’s towering 565-ft-high smokestack has shaped St. Paul’s skyline for decades, but the aging icon will soon face the wrecking ball. LG Constructors, a subsidiary of CH2M Hill Cos., Denver, is the engineering-procurement-construction contractor for a new $428-million, natural-gas-fueled, combined-cycle plant. The replacement facility, also on the riverfront, will generate 580 MW, more than double the current plant output, or enough added electricity for 270,000 homes.
Work is ongoing on the 110-ft-tall, steel-framed, metal-paneled building, which uses structural steel H-shaped beams up to 110 ft long and 3 ft deep.
Photo courtesy of Xcel Energy by Rick Souther
The High Bridge plant originally went into service in 1923 with two units feeding power directly to the city. The facility once formed the electrical hub of Northern States Power Co., a predecessor to St. Paul’s Xcel Energy Inc. The plant added two more units, in 1942 and 1944. The fifth and sixth units came on line in 1956 and 1959. Although the four oldest units have been retired, two boilers still provide process steam to a nearby paper manufacturer, delivered through a 5-mile pipeline built in 1983.
The 270-MW High Bridge plant will remain in place until its replacement is up and running in May 2008. Demolition will follow from June 2008 to December 2009. Although the old plant maintains some environmental controls such as burning low-sulfur western coal and using electrostatic precipitators that trap fly-ash particles, it can’t compete with its successor.
“The new plant will reduce harmful nitrogen oxide emissions by 96.9%,” says Patti J. Nystuen, an Xcel Energy spokesperson. “It will also decrease particulate emissions by 91.5% annually.
Xcel Energy’s move toward more environmentally friendly facilities is part of its Metro Emissions Reduction Project. In May 2002, the company unveiled a $1-billion plan to slash air emissions at three of its Twin Cities coal-powered plants, including High Bridge. Under MERP, the utility will install new emissions control equipment at the 529-MW Allen S. King plant in Oak Park Heights, Minn., and convert High Bridge and the 515-MW Riverside plant in Minneapolis from coal to natural gas. project will also increase the King plant’s capacity by 60 MW and Riverside by 80 MW.
The High Bridge project brings high risks because work is being done around an existing, operating powerplant. A portion of the 7.5-acre property also is being beefed up to support a new building housing 2.5 million lb worth of electrical-generating equipment.
The muddy site at 501 Shepard Rd. in Ramsey County first required $4-million worth of soil remediation and preparation, including the removal of a buried, concrete gas tank from the early 1900s. The site originally held a coal-gasification plant that supplied gas for St. Paul’s street lights before it was replaced by High Bridge.
After excavation and placement of 128,000 cu yd of fill raised the property to the required 100-year-flood level of 710 ft, specialty contractor and AGC of Minnesota-member firm Veit Cos. of Rogers, Minn., drove 1,194 H-shaped steel pilings 100 ft into the ground using a vibratory hammer. The 12-x-12-in. beams came in 50-ft, 2,650-lb pieces that were butt-welded together onsite into longer sections using Lincoln Electric Co.’s Vantage 400 engine-driven, diesel-powered welder/generators. The generators enabled continuous stick welding without cool-down time, increasing productivity by 20%, say project officials. Pilings were placed 3 ft apart on center across the building’s two-acre footprint.
“We drove 22 miles worth of steel into the ground in 10 weeks without a scrape or a single boo-boo,” says Eric Pederson, Veit’s foundation division manager.
An artist’s rendering shows the proximity of the new High Bridge plant to the Mississippi River. It will sit on 7.5 acres near downtown St. Paul.
Courtesy of Xcel Energy.
A 6-ft-thick, reinforced-concrete mat ties into the pile caps for a stiffened, level foundation that can withstand the turbine loading. The new plant has strict operating tolerances down to the millimeter. It’s equipped with a computer program that shuts down generators if even a slight vibration occurs.
The new plant’s generating equipment is housed inside an 11-story, 130,000-sq-ft building. Similar combined-cycle plants usually are designed with the generating equipment exposed to the elements but a sheathed plant helps protect the machinery and prolongs its lifespan. St. Paul winters can be brutally cold, with temperatures commonly dipping to -20 F.
“A covered plant creates a less conspicuous presence, which is something neighbors wanted due to the recent development activity in the area,” says Roy Snover, LG Constructor’s project director. “The new plant additionally covers half the amount of space as its predecessor. And it will have two 160-ft, 19-ft-dia exhaust stacks that are one-third as tall as the present tower.”
That’s important because St. Paul Riverfront Corp. is helping to redevelop the downtown property with 40 new mixed-use residential, retail and cultural projects. The $170-million Upper Landing project, for example, is being developed by Centex Multi-Family Communities LP in the shadow of the High Bridge plant’s stack. About 600 rental and owner-occupied houses opened in 2004 on a 21-acre riverfront tract by the plant.
PROJECT DETAILS > Cost $428 million ($394 million for design, construction and equipment; $34 million for soil remediation, site preparation and old plant decommissioning) > Building Size 130,000 sq ft > Building Height 11 stories (110 ft tall) > Work Force 400 (at peak of construction) > Backfill 128,000 cu yd > Concrete 19,000 cu yd > Piles 1,194 > Pile Depth 100 ft (average) > Wiring 6.5 million ln ft of single conductor line > Piping 70,000 linear ft > Gas Turbines Mitsubishi 501F > Turbine Weight 560,000 lb each > Boiler Feed Pumps Four 3,000-hp motors.
PROJECT SCHEDULE > Secured necessary permits January—July 2005 > Awarded major contracts July 2005 > Preliminary site preparation August 2005 > Mobilized turnkey contractor February 2006 > Earthwork and equipment foundations February — Oct. 2006 > Major equipment delivery June 2006 — June 2007 > Install mechanical equipment Spring 2006 — Fall 2007 > Startup and testing September 2007 — March 2008 > Commercial operation May 2008 > Demolition and removal of old plant June 2008 — Dec. 2009
ENVIRONMENTAL BENEFITS
The High Bridge project will reduce annual air emissions by the following amounts:
SULFUR DIOXIDE (SO2) 99.7%
NITROGEN OXIDE (NOX) 96.9%
PARTICULATES 91.5%
MERCURY 100%
The river played an important role in landing the plant’s generators, which came to the project in a roundabout way. Xcel purchased two Mitsubishi 501F gas-fired turbines that Atlanta-based Mirant Corp., already had partially installed at its new 550-MW powerplant in Wyandotte, Mich. But after attempting to restructure its debt, Mirant filed for Chapter 11 bankruptcy protection in 2003. Xcel purchased the turbines from Mitsubishi and the equipment was disassembled and shipped to St. Paul, arriving onsite via a river barge.
“The turbines are incredibly heavy, weighing 560,000 lb each,” says Scott Eddy, Xcel’s senior superintendent of construction. “The turbines are similar to a jet engine but are about four times larger.”
The turbines arrive onsite disassembled in thousands of pieces. It takes nine months to a year to pipe weld and piece them together, but it would otherwise take another year to order the turbines new from the manufacturer and six months more to receive shop drawings.
The project requires roughly 70,000 linear ft of piping and 6.5 million linear ft of single-conductor wire installation. It will employ 400 tradespeople during the peak of construction activity. Pioneer Power Inc., St. Paul, is the piping subcontractor; Azco Inc., Menasha, Wis., is the mechanical subcontractor; and Knutson Construction Services, Minneapolis is concrete and underground pipe contractor.
Meanwhile, workers are building the 110-ft-tall, steel-framed, metal-paneled building, which uses structural steel H-shaped beams up to 110 ft long and 3 ft deep. The building additionally houses offices for 20 employees, plus control and instrument rooms. There is a two-hour concrete masonry firewall between workers and electrical generating equipment. Calgary-based ATCO Group is providing the building shell. Merrill Iron & Steel Inc., Schofield, Wis., supplied structural steel, which was erected by Shakopee, Minn.-based Danny’s Construction Co. Inc. The turbines will be lifted and lowered through the roof using a Manitowoc 2250, a 500-ton crawler crane with 175 ft of boom. The turbines will be picked and set in four modules to form two power trains, placed 130 ft apart and secured with 2.5-in-dia, 2-ft-long bolts.
Exhaust from the gas-fired turbines is captured by heat-recovery generators that create steam for a combined-cycle operation that is 30% more efficient than a traditional steam plant, Eddy says. It’s cooled by Mississippi River water through a pair of 84-in.-dia 1,400-ft-long pipelines, one for supply, another for return.
Plant construction is currently on schedule for startup in May 2008 and so far is accident-free. Contractors have logged in more than 300,000 staffing hours to date without a lost-time incident, Snover says.
“We spend a lot of time coordinating with subs so that everyone understands what is happening onsite,” he says. “That’s why we’re so proud of the project’s track record to date."
