The Minnesota Pollution Control Agency is taking comment until Nov. 2 on an air permit renewal and permit changes that over a severe reduction of operations of the Silver Lake plant of Rochester Public Utilities.
Silver Lake is an existing steam generating station consisting of two natural‐gas fired boilers and one small natural‐gas fired heating boiler. Steam produced at Silver Lake is sold to the Mayo Clinic via a high‐pressure steam line to Mayo’s Prospect Plant, where it is used to generate electricity via a stream turbine, with the waste heat used for building heating. The total amount of steam sold is limited to 150,000 pounds per hour (lb/hr) on a 12‐month rolling average basis.
Silver Lake was previously permitted to operate all four boilers (EUs 001–004) on coal and/or other fuels. The boilers were used for electrical generation and steam service. The facility ceased coal firing permanently in 2013.
The agency noted that two of the boilers (EU001 and EU004) have ceased operation and will be officially retired by the end of 2015. Silver Lake no longer produces electricity for sale and will operate its remaining units on natural gas only.
Due to these changes, the MPCA sought and received verbal approval from the U.S. Environmental Protection Agency to remove all SO2 and PM10 State Implementation Plan (SIP) requirements from the Silver Lake permit, and add requirements to use natural gas only at the remaining boilers. Once this revision is approved by EPA, the SIP‐allowable potential‐to‐emit (PTE) for Silver Lake will go from 6,220 tons per year (tpy) to a scant 1.12 tpy of SO2 and from 2,060 tpy to 14.2 tpy of PM10.
Said the Rochester Public Utilities website about the Silver Lake Plant (SLP): “As of June 1, 2015, SLP is a steam producing facility providing a contracted amount of steam to the Mayo Clinic campus for cogeneration needs. The fuel burned for steam production in the boilers is natural gas. Prior to the transition from an electric generation facility, SLP was a 100-megawatt, coal-fired generating facility. The four boiler/turbine/generator units varied in age and size ranging from Unit #1 at 7.5 MW (1947) to Unit #4 rated at 55 MW (1969). Pulverized bituminous coal was the primary fuel, and was supported by natural gas.”