This May, when Xcel Energy’s High Bridge Combined Cycle Project entered commercial service, it marked the end of an era in St. Paul, Minn. The new 570-MW natural gas-fired plant, on the banks of the Mississippi River near downtown St. Paul, replaces the 270-MW coal-fired High Bridge power plant built in 1923. Like the old plant, the new one takes its name from the structure that spans the river nearby (Figure 1).
1. Before and after. Xcel Energy’s new 570-MW gas-fired High Bridge Power Project (top) has replaced the utility’s 270-MW coal-fired plant (bottom), adding 300 MW of capacity and substantially reducing the site’s air pollution and CO2 emissions in the bargain. The view is to the west. Courtesy: CH2M HILL
In 2003, rather than wait for federal air pollution mandates to kick in, Minneapolis-based Xcel Energy proactively opted to scrap the old coal plant in favor of cleaner and more-efficient gas-fired generation technology. At the project’s groundbreaking ceremony in May 2006 (see table), David Wilks, president of Xcel Energy Supply, explained that, “Last year, Xcel Energy kicked off its voluntary Minnesota Metro Emissions Reduction Project, or MERP, and this plant is a key part of it.”
High Bridge Combined Cycle Project timeline. Source: CH2M HILL
In addition to building the new High Bridge plant, the MERP also aims to significantly reduce air pollution from two other Twin Cities coal-fired plants while increasing the capacity of one of them, at a total cost of approximately $1 billion. As part of the plan, Xcel has installed state-of-the-art emissions control equipment at its Allen S. King plant in Oak Park Heights, Minn., and is in the process of converting its Riverside plant in Minneapolis from coal to natural gas. The MERP is scheduled to be completed by May 2009.
The High Bridge Project will reduce air pollution from the site by the following amounts: SO2--99.7%; NOx--96.9%; particulates--91.5%; and mercury--100% (unlike coal, natural gas contains no mercury).
The plant has a conventional 2 x 1 combined-cycle configuration; each of its two Model 501F combustion turbines built by Mitsubishi Heavy Industries exhausts into a heat-recovery steam generator (HRSG) from Nooter/Eriksen, and the outputs of the HRSGs are combined to feed a Model TC2F steam turbine-generator, also from Mitsubishi.
High Bridge generates all of the hydrogen required for operations on-site using a Proton Energy Systems hydrogen generator that makes hydrogen from water. As did the old coal plant, it uses river water for once-through cooling, eliminating the need for a cooling tower and the possible generation of unsightly and unsettling “blue plumes.”
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