St. Clair Power Plant

Units 1 – 4 are 163 MW Babcock & Wilcox boilers tied to GE and Allis-Chalmers steam turbines. These are the four original units at St. Clair. Generating units 1–4 are St. Clair's base load units, usually running at full capacity. The flue gases from those units exit through south stack, which was erected in the late 1970s when new electrostatic precipitators were added to these units. When the plant was first built, there were four relatively short stacks, one for each unit. St. Clair Unit 4 retired early, in 2017, due to mechanical problems and Unit 1 was retired in March 2019.

Unit 5 is St. Clair's first decommissioned unit. Its cyclone boiler produced 300 MW, and was taken out of service in 1979 due to a mechanical problem with the boiler. The stack for unit five was removed in 2012.

Unit 6 is a tangentially fired Combustion Engineering dual furnace boiler tied to a Westinghouse turbine. Commissioned in 1961, this unit produces 321 MW.

Unit 7 is also a tangentially fired Combustion Engineering boiler tied to a Westinghouse steam turbine. It was commissioned in 1969. Unit 7 is rated at 451 MW, originally built to produce over 500 MW. This unit is St. Clair's largest generating unit, but has a small electrostatic precipitator, causing problems with the opacity when burning western coal.[7]

An oil-fueled gas turbine Unit 11, rated at 18.5 MWe, was added in 1968. Two smaller oil-fueled internal-combustion generators, totaling 5.4 MWe, were added as units 12A and 12B in 1970.[3]

St. Clair burns a blend of low-sulfur Western coal (from the Decker (Powder River Basin) and Spring Creek mines in Montana) and high-sulfur Pittsburgh-seam Eastern coal. Blending is done in the coal yard with variable speed feeders. Although the nominal blend is 85% Western and 15% Eastern coal, the exact blend depends on a number of factors including demand requirements, coal prices, stockpile inventory, and unit availability. In the summer months, for example, the blend trends toward 70/30 on Units 6 and 7 to gain additional generating capacity by firing more Eastern coal with its higher heating value. The higher sulfur content of the Eastern coal also helps prevent opacity excursions with the electrostatic precipitator on Unit 7.[7]

The Western coal is delivered to Superior, Wisconsin, by unit train and then barged to St. Clair on 60,000-ton ships. Because the ships typically cannot operate on the Great Lakes from late December until late March, the plant must stockpile between 2.1 and 2.4 million tons for the winter season. Eastern coal is delivered by rail to an unloading facility. Because of decline in production, the plant's seasonal stockpiling of Eastern coal declined from 140,000 tons to 20,000–30,000 tons in 2004.[7]

With its oldest unit dating back to the 1950s, the plant was ranked 74th on the United States list of dirtiest power plants in terms of sulfur dioxide emissions per megawatt-hour of electrical energy produced in 2006. Sulfur emissions could be lowered by using flue-gas desulfurization units, better known as SO₂ "scrubbers", like those of Lambton Generating Station across the St. Clair River. Currently, these scrubbers are being installed at DTE's Monroe Power Plant, and may eventually be added at the St. Clair site as well.

All of the waste heat generated by the plant (about twice its electrical output) is released into the St. Clair River. This water is used in large condensers to cool the used steam back to its liquid form. Water used in the condensers is treated, filtered, and replaced back into the river. Other water used on site for cleaning boilers and to drive the turbines is treated in settling ponds where ash and particulate are suspended from the water. The water is purified and pumped back to the river. The particulate is taken to the St. Clair/Belle River ash landfill a few miles north.