Xcel Energy’s Riverside Repowering Project was a voluntary emissions reduction project with the goals of improving air quality in Minnesota while, at the same time, increasing the amount of electricity produced. The Riverside Plant was converted from being a coal-fired facility to a combined-cycle facility firing natural gas. The fuel switch resulted in significant plant emission benefits, with emissions of sulfur dioxide decreasing by 99%, nitrogen oxides (NOx) decreasing by 96%, and mercury being eliminated entirely. Plant output increased from a nominal generating capacity of 386 MW to a summer day net output of 472 MW. The three existing coal-boilers were retired. Two F-technology combustion turbine generators (CTGs) and two heat recovery steam generators (HRSGs) were installed. Each CTG fires natural gas with dry low-NOx combustors. Each HRSG includes a selective catalytic reduction (SCR) section supplied with aqueous ammonia. Steam generated from the HRSGs is fed to an existing steam turbine. The low-pressure steam from the HRSG is admitted into the steam turbine through a new connection. The steam turbine extraction lines for feedwater heating are capped. To accommodate the resulting increased flow through the turbine, two rows of blades were replaced. A new full-flow steam turbine bypass system operates during plant startup and shutdown. An auxiliary boiler was added to provide warming and sealing steam to the steam turbine. A new distributed control system (DCS) operates the facility, with workstations located in the plant’s existing control room. The existing once-through circulating water system on the Mississippi River was modified with the addition of wedgewire screens to comply with the Clean Water Act Section 316(b). The CTGs and HRSGs are fully enclosed in a new building that is integrated with the structure for the steam turbine. Due to the proximity of residential housing, sound attenuation is critical. Space for the new building was created by demolishing formerly retired units. Reclaiming this area resulted in a unique layout. The HRSGs are constructed over wood piles installed in 1914–22. The HRSG foundation consists of an elevated slab supported on concrete walls distributing load to the original pile caps. Between the HRSGs and the CTGs are retired concrete coal hoppers that divide the site. The CTGs sit approximately 20 feet higher than the base of the HRSGs. The combined cycle achieved commercial operation on May 1, 2009.

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